Remember these rules:
1)the BEST seat is the one that fits your child, fits your car, and will be used correctly 100% of the time. (This is why convenience features DO make a difference and ARE worth the money! If its easy to use, you're more likely to use it correctly.
2)NEVER buy a used car seat!
3)Children should stay rear facing AS LNOG AS POSSIBLE!!!! The 20lbs/1 year rule is outdated and provides a bare minimum for turning kids forward facing. The American Academy of Pediatrics and the National Highway Traffic Safety Administration both recommend keeping kids rear facing as long as possible, up to the limits of their seat, preferably until at least 2 years of age. For good reason: A forward-facing child under 2 years old is 4 times more likely to be killed or seriously injured in a crash than a rear-facing child of the same age. A child's vertabrae do not fully fuse until 3-6 years old, before then, she is at great risk for internal decapitation. The spinal column can stretch up to 2 inches in a crash BUT the spinal cord can only stretch up to 1/4 inch before it snaps and baby is gone. In other countries, rear facing 2 - 3 - 4 year olds is standard, they understand that its safer. Here, we turn them as soon as we get to, seeing it as a right of passage thing or something. Ridiculous. Most convertible seats have a 30lbs rear facing limit, Cosco/Dorel/Safety1st/Eddie Bauer seats rear face to 35lbs, Britax rear faces to 33lbs.
4)Once you do turn them forward facing, they need to stay in a 5 point harness as long as possible. 4 years/40lbs is the minimum for riding in a booster, and most 4 year olds have no business using one yet. If they can't sit upright for an entire trip, they need the harness of a car seat still. And, even if they do sit properly, a 5 point harness is safer, so you want to keep them in one as long as possible. This is important to consider b/c most car seats only forward face to 40lbs. My son just turned 3, and is in the 95th% for height and weight - 40" tall and weighs 41lbs. He outgrew the 40lbs seats shortly after his 2nd birthday. It was a total waste of money. He now has a Britax Marathon, which goes to 65lbs, and will be able to fit it for some time yet. If I'd have bought it when he was born, I could have had one car seat this whole time instead of the 3 I wasted money on. They are more expensive for many reasons, this is one.

So...knowing all that, here's some about specific seats. Please don't get a Graco Comfort Sport. Car seat techs call it the 'crappysport'. LOL Its a crappy seat that won't last your child very long b/c it has a really short shell. The straps twist all the time, and if they're twisted they won't support your child in a wreck properly. They're hard to install. There's nothing good about these seats except perhaps they're cheap sometimes. Ditto the Safety1st 3-in-1/Cosco Alpha Omega/Eddie Bauer 3-in1 seats. These are all the same company - same seat, just different covers. They stink. Hard to install, b/c of narrow belt paths. Ever tried to wash a skinny cup by hand? Now imagine that skinny cup with pointy edges. That's what putting your hand through these to install them is like. AND they have too short a shell to really go to 40lbs. And they do not make good boosters. Also, most kids have to use boosters until they're at least 8 years old. Car seats expire 6 years after the date of manufacture, so no matter what they say it is NOT the last seat you'll ever need to buy.

Realize that convertible car seats are almost always outgrown by height before weight. This happens when either:
1)the child's ears reach the top of the shell forward facing
2)the child's shoulders go over the top harness slot
So, realize that just because a Britax Marathon has the potential to be used to 65lbs, does not mean that it will last your child until they are 65lbs. It completely depends upon how your child is built. Kids with long torsoes will outgrow seats long before kids with all their height in their legs. The average child will outgrow the Marathon at 5 years old/55lbs.  This is exactly why getting a 40lbs convertible car seat just usually isn't cost effective - it will be outgrown by height before the child reaches 40lbs, which means you will be looking for another harnessed seat to last to booster age. In the goal of buying as few seats as possible, look for high harness slots and high weight limits from the getgo and save yourself time, trouble, and money.

The Britax Marathon/Boulevard/Decathalon (These 3 are the same seat, just different variances of luxury additions, with the Marathon being the cheapest base model) can be found for regular price of $269, and on sale at Albee Baby online for $209-$219. The Marathon a wide open easy to route belt path, which makes using it correctly a lot easier, as do the built in lockoffs, which mean you never again have to use a locking clip. I can get it into 4 different vehicles in under 2 minutes. Because of the way the base is made, it fits in most cars. And, it is worth the money, b/c it lasts usually twice as long as most other car seats (all car seats expire 6 years after the date of manufacture) lasting a baby till they are 5-6 years old, where most at Walmart will only last till 2-4 years old. Rear faces to 33lbs, then forward to 65lbs. and top slots 17inches tall, lasts most kids to ages 5-6 years old.

The Evenflo Triumph Advance (not the original Triumph, make sure it says Advance) is a great seat. $150 version at Babies R Us has padding similar to Britax seats, top slots of 17", harnesses to 35lbs rear facing, and 50lbs forward facing. $120 Walmart version just has little less plush padding. Wide open belt path, easy to install, though it doesn't have built in lockoffs. The harness adjusts at the front of the car seat, you don't have to take the car seat out of the car just to raise/lower the straps. It's one of only 2 seats that does this (The Britax Boulevard is the other, I believE), and it has infinite harness adjustment so the harness always fits perfectly until its outgrown. No more tugging straps to tighten them either. You tighten and loosen the harness using knobs on the side of the seat. As a major bonus, it can be used in a recline position even in forward facing mode. Awesome for kids who still sleep in the car. I LOVE THIS SEAT! LOL My son, who is too big for every other car seat at Walmart has the same amount of room in this as the Britax Marathon.

The Fisher Price Safe Voyage Deluxe Convertible seat is great, it is made by Britax, without all the frills. It's the same shell as a Marathon, but for less than half the price. $130 at Albee Baby online with free shipping, or at their eBay name, babysurplus. Rear faces to 33lbs, forward to 55lbs. Easy to use, fits in most cars.

One that makes a great spare/temporary/travel seat is the Cosco Scenera. $50 at Walmart/Target/Kmart, shop around, each store has a different colored cover! It is a great rear facing seat, goes to 35lbs, though it is outgrown quickly forward facing because of short top slots - 14.5"! Its lightweight and easy to install, without any frills. Most kids outgrow it before they turn 3, though, so realize it will not get you to booster age.

These are the best you can buy that will give you the most bang for your buck. As a single parent I'm always watching the budget - I have no money to speak of, and these are what I would buy. (My son has a marathon and is soon getting a Britax Regent). They will last the longest. I only wish someone would've told me all this before I had him, I coulda saved some money for the extra seats I started out with!

All seats in the US must meet the same safety criteria to sell, so technically they all are safe, however, if its hard to install, then it won't be used correctly and that makes it unsafe. Also, they only must meet the guidelines, some seats barely pass, some exceed them. Britax exceeds the guidelines and is easy to use.