I agree with Mac regarding the 2.5" hard drives being tough as a result of the normal use in laptops. But I would also think that the 3.5" form factor external hard drives reliability would be determined by the housing it was put in. Some are more shock protected than others, and some have additional cooling methods (fan, fins or both) because the 3.5" generally run hotter. And you can get a very large capacity for a lot less per gb than currently available with the smaller drives. If you use the usb drive to run BIAB and other programs for gigging or moving around a lot, that will make them more susceptible to damage and/or failure than if they just sit next to your computer hooked up all the time. So don't discount the 3.5" drives, you can make them work quite well.
Good luck.