Edit: There's also the issue of in game loading. Some times fast loaders will speed up in game loading, and some times they won't. Not sure what the difference is, but I hear JD works consistently.
From what I have read, some programs are designed to disable any cartridges, or at least any fast load routines from the cartridge port. Since Jiffy is kernel level, it isn't typically disabled by programs. I've played with some of the other carts available and it always seems like I hit something that won't load. Jiffy seems to be pretty universal. I've gotten to the point where I use Jiffy for my fast load needs and keep the Retro Replay 3.8 image selected, but with it's fastloader disabled. That way I get the speed compatibility along with the freezer and other associated utils.