When choosing aftermarket Mustang gears (or any aftermarket mod), it’s important to consider what type of driving you do/want to do with the car, how much power you currently make, and how much power you intend to make.
Aftermarket rear-end gears are designed to get you off the line faster, so you’re going to burn through first gear (and second… and third…) much more quickly than with your stock gears. This isn’t as much of a hassle with an automatic transmission because you aren’t the one shifting, but if your Mustang is a stick and you do any kind of driving in a city or heavy traffic, you’d be wise to avoid the steeper ratios. Considering your pony is already super charged, I would have to side with your friends and recommend the 3.73 gears; they’ll give you the extra kick off the line that you want without making driving a chore.
When installing aftermarket gears, it’s always a good idea to get a bearing kit and replace the shims, bearings, and other items that wear out over time. This will complete your rear-end rebuild and make sure you keep that rubber burning for a long time.