I have to have both. FT got me into pinball and I've been to the race 9 times. I made the mistake of selling I500 once and it took me 6 years to get it back. Luckily it did not increase in price very much over that time. I always wanted to upgrade my FT to a nicer one, but never pulled the trigger, and now that the prices are so stupid it'll probably never happen. I think I paid around $1500 with shipping in 2006, which was wayyyy too much for back then considering the condition it was in. I might actually be able to turn a profit if I sold it now, but that's not happening!
As far as gameplay, I give a slight nod to FT. Yeah, it's not as deep, but there's is something cathartic about hitting those boat ramp shots and then nailing the monster fish. The video mode is way better than I500's too. I like to drink a beer (or six) and not have to think too hard when I play pinball, so the depth of a game isn't necessarily a big attracter for me.