It comes down to preference of how the FFB feels, plus a couple of other things.
T500:
- fixed paddles, 30cm diameter rim.
- 25% more powerful FFB.
- more resistance turning the wheel, less easy to flick the wheel around holding slides (holding slides is obviously possible, just more challenging than T300 and probably more realistic)
- nice progressive feeling of weight, big dynamic range of forces.
- proven technology, good reliability.
- not full PS4 support (works in all current and announced future games but not in menus)
T300:
- moving paddles, 28cm diameter rim.
- 25% less powerful FFB.
- less resistance turning the wheel (comparable to a G25/27 but without the cog feel), very easy to flick the wheel around to drift and hold slides.
- lightness of the wheel feels less progressive than the T500, but on the other hand you can feel smaller forces better (more "nuance"). Similar dynamic range but it starts softer and goes less "loud" than the T500.
- still quite a few guys buying new T300s and having issues, so if you are unlucky you might have issues.
- full PS4 support.
I use my T300 when I want to drift or drive fast modern race cars, or when looking for the absolute fastest laptimes (most control). I use my T500 when I want to run road cars, rally cars, or vintage race cars.