A:
Shorter one will work just fine. Yes the longer one will help more with heavier bows with higher draw weights or heavy vibration, but will have to be removed to case the bow. Your shot groups will barely have a noticable diffference between the two. Both will signifigantly decrease noise and vibration.
I use the 7" version as I do not mind removing it to case the bow. But the performance difference was minimal. I gave the 4.5" version to my wife and got myself a 7" version. Both work as advertised, with the 7" version my groups are TIGHT, with the 4.5" version I might have been 1/16th of an inch bigger in my groups. Negligable performance increase for an increase in inconvenience. If I had to do it again I'd have stayed with 4.5" I would still kill the deer equally as dead, and still hit the target in the bulls eye. But that extra 1/16th of an inch of confidence is staying on my bow since I already paid for it.