I do love the FX5. It's got some nice extra features that the others don't. Plus, it would have no problem upgrading to a 125 gal or even larger.
The XP3 is a good filter as well, especially for the price. I've heard good things about it, but never used it myself. It might be able to squeek by in a 125 gal, but you may end up needing a secondary smaller filter to keep up.
I don't know very much at all about the Eheims, but I hear they're quite good as well.
If it were me, I'd go for the FX5. It'll be overkill in a 55 gallon, but I've heard of a couple people using it in a 55 gal before, so it's not unheard of. You can also turn down the flow, and according to Fluval's website, it doesn't decrease the life of your motor.
Also, I would recommend upgrading to something larger than a 55 gal if you can. It's really already tight for two turts, and they'll outgrow it quite quickly. If you can't afford it right now though, it should be fine. Just keep squirrelling away your change, and you'll have enough for a monster tank in no time
I don't know how much experience you mom has with turts, but maybe you could encourage her to check out this website.
At the very least, teach her about the signs of aggression, so if it comes to it, she can separate your turts if they start fighting. Otherwise, one or both could get seriously injured, and that would be a much worse situation!
Good luck in college!
