Now that I own both the T300RS and T500RS, I'd say:
T500RS: Gives nice powerful FFB, but has a bit more internal resistance so you need to work harder to flick the wheel around. Far better pedals. I like the 30cm rim and fixed paddles, but for some people the lack of moving paddles will be a disadvantage (28cm GTE rim @ €80 is the solution but pushes the price up).
T300RS: Slightly weaker FFB (about half way between G25 and T500RS in power), easier to flick the wheel around than the G25. Very smooth, very quiet, very good at telling you about nuances of grip, good dynamic range even if not as brutal forces as the T500RS. Crappy pedals (similar in quality to DFGT but with metal faces and firmer springs, so actually a step up from DFGT pedals if you're coming from those)
The T300RS probably has more in common with the G25/G27 (lightness, easy to flick around) than the T500RS in use. But then a considerably improved G25/27 with no cog rattle, faster spin speed and better dynamic range.
Given that you can get a T500RS or a T300RS+Ricomotech G27 pedal adapter for nearly the same price right now, I'd say the T300RS+G27 pedals is probably the better option. It really is a very good wheel.