I have both, and prefer the T300: it's smoother and more agile than the T500, which more than makes up for the less powerful motors. If you don't need the pedals and don't like fixed paddle shifters the T300 is the better wheelbase...
Yes, the T300 appears to be still having some early teething troubles, so there is a risk that the next one also has problems... And I would also understand wanting maximal FFB power which the T500 will give. But if you want versatile and agile force feedback where you can feel the smallest sensations and still get decent powerful forces then my vote goes to the T300.
If you get the T500, just make sure it has a GT6 branded box. That's the latest release. Later than the v2-v5 stamped GT5 box versions.