The G25 is slightly old, but is commonly accepted as the standard to which all steering wheels aspire. Its clutch and shifter add a measure of realism to GT, the metal steering wheel is unfettered by buttons, and is given great, accurate force feedback. There are a number of simple mods that can be done to the G25's shifter to improve it dangerously near the standard of perfection, and aren't very risky.
I won't pretend to know anything about the practical application of the 911, except for this: It has belt-driven force feedback, which means there is less lag, but it may be noisier than the gear-driven force feedback used in G25.
I don't know if the 911 is better, but you certainly won't be discontent with a G25 in any scenario. But, with something this expensive, you should only take the advice of someone who's working only with theory (me) if you're left with no other option. Best of luck to you!