Logitech shifter doesn't work on T500RS so you'll have to buy TH8 shifter (when it finally gets available).
As for the question...
G25 is an old wheel, you should deffo look at something new.
T500RS or a Fanatec wheel? Depends on what you need from the wheel and how big is your budget.
What you play? PS3 only? PC & PS3? PC, PS3, 360?
If you're playing PS3/GT5 only - Fanatec is the wheel to go for.
GT3 RS ClubSport Edition, you can't go wrong.
If you're playing PC sims - get T500RS with CSP (or wait for Fanatec's CSW).
If you have a big budget I'd advise you to wait a bit more for Fanatec's ClubSport Wheel to come out, if not just get either GT3 RS or T500RS, depends on what you want.
T500RS
+ Better FFB
+ Metallic feel
- Price
- No shifter (yet)
GT3 RS CSE
+ Amazing tuneability
+ ClubSport Pedals
+ H shifter
+ Less expensive than T500RS
T500RS is a better wheel, GT3 RS CSE is a much better all-round package (again, if you only use it on PS3).
The point I'm making is that T500RS is an amazing piece of kit but it just can't be used to it's full potential on PS3 because it lacks some vital tuning fuctions which are available on the PC, also, on the PS3 it can't be combined with 3rd party hardware (such as CSP), that's why I wouldn't recommend buying T500RS just for PS3.