OK, you've added a cold air kit, maybe changed throttle bodies or added heads, cam and intake -- maybe a blower?  Do you now have a surging idle or poor part throttle drivability?  It could be due to your MAF sensor (or some other things that I will describe in depth in a bit).  Whatever the reason, you need to know how to properly choose one.  I'll go through some tips and suggestions that should help you to narrow down your decision.

We should probably go into very basic MAF function and how it relates to the various changes you make to your vehicle.  Our beloved Mustang has been using this technology since 1989 in most of the country (CA cars, since 1988).  If you have a pre '89 car and tried to do any extensive modifications, you know that the speed density system is not as adaptive as the MAF system.  You'll want to convert your early vehicle to MAF if you plan on doing modifications beyond headers/exhaust and maybe a CAI kit.  But I digress...Basically, the MAF sensor uses a 'hot wire' that is dangled into the intake stream.  The hot wire has a current flowing through it to heat it to a specific temperature.  With that heat, its resistance is increased (ever have a problem starting a car with a hot starter, heated by close running headers?  That's due to the increased resistance).  When the air is rushing by (when your throttle is opened) the wire is, of course, cooled and will have its resistance decreased and thus will allow more current to flow through the wire, heating it up to the constant temperature required.  The electronics on the meter will then convert that measurement into a signal the computer can understand.  Armed with this info, as well as some info from other sensors, the computer can now make adustments to the fuel side to compensate for increased airflow.  Why this works so well and is so versatile is, when the ambient temperature changes or you have increases in pressure (blower, turbo etc.), the density of the air increases.  With this system, the computer can compensate for this change -- up to a point.  Again, this is a very basic explanation on MAF function. Now, on to the decisions you'll need to make.

The first question you should ask yourself is, how far will you be going with engine modifications to your Mustang.  If you're just going with a CAI kit and maybe a throttle body, well your choice will be different than a car with heads, cam, intake and turbo kit.  Unfortunately, if you're like me, you always start out with a goal of mild and the project turns into wild after you're done :)

The second question (prettty much determined by the first question) you would want to think about is, will I require a tune and, if so, who and how will it be tuned?

Let's talk about manufacturers and the differences between them.

C&L Performance
C&L has been making housings since the early 90's.  Their housings for the Fox & 94-01 SN-95 cars were designed to use the OEM hot wire electronics but change the amount of air flow going by the hot wire and thus tricking the OEM computer programming into figuring out how much fuel should be added, while still thinking that nothing in the intake tract has changed.  The products they make for the later Mustangs (02-08) use the OEM sensor but just drop into the new C&L MAF housing.

These housings have to be handled seperately as they work differently for the 02+ cars.
The Fox and 94-01 SN-95 housings (73mm, 76mm, 80mm) are great for mildly modified cars as they are sort of a brute force way of adjusting things.  As mentioned earlier, the sample tube orifice diameter is changed according to your injector size.  This will limit the air going past the hot wire, changing the reading it will give the computer.  They work well for a general range of airflow.  If you do start going into forced induction, I do not recommend this meter.  They otherwise work well and will allow for increased airflow matching an upgraded intake tract (CAI kit, etc.).

The 02-04 cars use the OEM electronics as well but do not need to use the sample tube.  The 05+ cars also use the OEM sensor but require a tune as they are extremely sensitive to changes in the intake -- no matter who makes the MAF housing or CAI kit.  These 02-08 MAF sensors and intake kits are great for just about any combination, including supercharged.

Professional Mass Air Systems (PMAS)
PMAS makes specifically calibrated meters and are a more accurate way of adjusting the signal sent to the ECU to get the desired results.  With a PMAS meter, you'll get new, properly calibrated electronics installed in the housing of your choice.  If you desire, you can also get the MAF transfer function which will help your dyno tuner get the proper info into the tune they are building.  In forced induction applications, PMAS standard meters can be run in draw through (before turbo or blower) or blow through (after turbo or blower) when requested during the ordering process.  For added accuracy, which will help with drivability and idle quality, these meters can be calibrated for use with CAI kits and blowers or turbos with piping, that may bend to various degrees, ahead of the meter.  It's a great option for basic and highly modified vehicles but it is more expensive than the C&L housing.  As every application is slightly different, these meters are calibrated to the application, using a flow bench, and will require the buyer to supply as much detailed info about the combo as possible.  Injector size, induction type, manner in which it is being run, engine size, inlet tubing type & tuning method, if any, are all required to get the most accurate MAF possible.  If you ever change your application, you can send it back to PMAS for a recalibration, so you're not going to be out of luck if you change things.  For tuning savy racers, they also have a drop in style 6 pin meter called the HPX.  It allows you to adjust the injector size and such in the supplied software and spit out the proper info for using with your tuner.

SCT
SCT has their new line of Big Air MAF sensors (made by Granatelli).  This meter is often requested by the dyno tuners using SCT's Advantage III software as it's really easy to calibrate to the injector size (in the software) as opposed to entering data from a chart.  There's not a whole lot to be said about these meters except they work well but are not plug & play like the PMAS meters.  They come in 2 sizes (2600 CFM & 3000 CFM) and are flanged but can be used with the fox cars (just knock off the flange).

While there are many more companies that produce MAF metersand this is by no means a total guide to MAF meters, these 3 are the major companies out there.  Each manufacturer's meters have applications where they'll work best.  If I can answer any other questions, don't hesitate to contact me!