These cars are light so whatever 'rated horse power' a trans may have can easily be increased when used in a Cobra. Of course you CAN break anything if you try hard enough.
I would base MY choice on the ability to get the GEAR RATIOS I wanted in ALL the gears, including that all important 5th OD. Which, in my opinion, would HAVE to be a .8 If the trans does not offer a point 8 gear range, scratch that one off the list.
The fundamental problem with a T5 is the ultra low 1st and 2nd gear ranges. Wheel spin is a serious problem in Cobras and having to low a 1st gear is a VERY common complaint. Followed by having to high a 5th gear, typcially .6 in a T5 with an ultra low 1st gear!
I would like a five (or six) speed trans to have a 1st gear similiar to the same range used in a wide ratio top loader. This is ESPECIALLY true if you plan on a rear gear lower than 3.5, which is all ready borderline to low. Of course a lot of this depends on your engine, small blocks generally need a lower gear, for instance. Be sure to give some serious thought about TIRES to address the traction issue, big or small block.
Finally you HAVE to consider your cam profile when choosing your gear ratios, particular the final gear, 5th overdrive lets say. If your cruise rpm is around 1800-1900 due to a HIGH OD gear combined with a high rear gear BUT your cam likes 2000 or higher, you got a problem. Cruise rpm should fall well within range of where the cam likes to work. This will maximize mpg, mean quieter sidepipes at speed, be less wear and tear on the engine and make the driver a happier person!
Choose wisely my friend, it's tough to 'live with' gear ratios you really don't like. Your not likely to change them later, to much hassle, to much money, so you 'live with it', not a good plan.
Heres the gear ratios for a WIDE ratio top loader. Running these gears with a 3.5 or 3.3 would be just about perfect for most replicas. NOW, throw in a 5th gear OD at .8 and you've really got a nice setup! Of course, thats just my studied and well thought out opinion.
Wide: 2.78 1.93 1.36 1.0 Generally speaking, around 35-40 mph in 1st gear.
Close: 2.32 1.69 1.29 1.0 Generally speaking, around 55-60 mph in 1st gear.
You DON'T want 25 mph in 1st gear or you WILL fry the tires without mercy.
Quite a difference in the close ratio 1st gear, pretty high for a small block, even a big block without due consideration of tire size, rear gear and cam.