My son who is now three outgrew his infant seat at 3 months. This did not come with latch three years ago, but is absolutely fantastic. I used with the top tether always in my lexus truck, and used it rear facing until he was a year old. It never budged once installed, and now I am buying a britax husky to take him to 80 lbs. In retrospect, had it been available then, I would have picked the marathon which goes to 65 lbs, and I could have just switched to a lightweight booster at that point. It is still really nice to have a small, light, and portable infant car seat for the first few months, when you don't want to disturb baby every time you pop her in and out of the car. So you might want to buy the marathon when baby is 6 months or heavy enough you won't be carrying around a car seat with baby strapped in it :-) And depending on the state you live in, the marathon might be the last car seat you ever buy.

And Britax makes the absolute best car seats you can buy -- my son has always been comfortable, and he always loved the britax galaxy infant seat over the evenflo i had for my husbands car.
He has been doing great in the roundabout the last three years too.

To make one more comparison, the Husky which goes 22 to 80 lbs is really only for an over 1 year old since it does not install rear facing. It is also an enormous and heavy seat which works fine in my truck, but seems a bit big for the Audi A4. So I would still choose the Marathon over the Husky if we were to have another child since it is sized similar to the Roundabout yet takes you to 65 lbs. If you don't have latch, absolutely choose Britax, since it is the safest seat out there and installs just as firm with the top tether as any seat with latch does!