I switched from a G29 to a T300RS GT, and the upgrade is easily worth it. At the end of the day you have a warranty, so just buy from a place with a good return policy.
Regarding the differences between the two, reasons why I prefer the T300RS GT.
1. It is a much smoother wheel. You feel less grinding or gear friction, which in turn makes it feel closer to a real wheel.
2. The force feedback is stronger.
3. The force feedback is more accurate. When you're on the edge of grip with the G29, often the way the gears function, the feedback drowns out a touch of the finer detail and nuance, whereas the T300RS is better at informing you when you've pushed too hard. The way the belts tug to one side, replicates a tire slipping or losing traction more accurately than the knocking about or gear grinning inside the G29.
4. The wheel is quieter with the FFB, though the fans can be a nuisance.
5. I prefer using the T300RS rim, as it feels larger and your hands have more room to manoeuvre due to less thick handles. The G29 however has premium leather whereas the T300RS uses simple rubber grips.
6. The paddle shifts have a nicer more chunky feedback.
7. The T300RS GT brake pedal is nicer to use with the conical mod it comes with. It's still stiff, but not too stiff like the G29 brake can sometimes be. The modulation seems just a touch more accurate, and the pedals themselves are wider too.
There are things I prefer with the G29, like the button mapping on the rim, the look and materials of the pedals, the reliability and the leather on the grips etc, but overall the T300RS GT is well worth the upgrade and just a more realistic, satisfying wheel to use.