Some of the pros attributed to belt driven wheels is simply because they're a better (and more expensive) wheel. For example, most (if not all) belt driven wheels have a higher peak torque than gear drive wheels - higher peak torque is a good thing but this is down to the manufacturers choice of gear/pulley ratio and motor(s) to use rather than being specifically attributable to a belt is better. With that said, gear driven wheels and belt driven wheels work on the same principle of a small motor with a small gear or pulley driving a larger gear or pullet attached to the wheel. This means they suffer most of the same cons; lower FFB torque, slower rotational speeds and some tiny torque inconsistencies due to gear lash and belt tension when FFB oscillates. As said though, these cons apply to both though so can't really judge which is better on these. There is one advantage belt wheels have; smoothness and as a consequence, noise. Because of gear lash (the small gaps between the gear teeth) when the FFB engages and closes this gap there can be quite a clunk noise and over something like a kerb that causes oscillation of the FFB it causes a rattle. As a belt pulley is designed differently, there isn't the requirement for this lash and so are quieter in their operation.
With respect to your specific question, objectively the T300RS GT is a better wheel; as above it's belt driven so will be quieter (though the fan on the TM still makes some noise), a quick google shows the peak torque being almost twice that of the G29 (3.9Nm v 2.1Nm), the T300 is 1080 vs G29's 900 degree rotation, there's more customisation options with the TM as you can change wheels rims if you want as well as being able to expand the TM to include a h pattern shifter (which is far superior to the Logitech offering) and a handbrake/sequential shifter if you want one too. Finally, the t3pa pedals are better than the Logitech offering too - this is arguably subjective rather than objective but it's hard to find someone that says otherwise so I'm counting this as objective too.
How much you 'value' you attribute to all of that is your personal opinion and somewhat governed by what you like to race. In my opinion though, the price difference isn't insignificant but it is worth it.
TL;DR - Is it worth the price jump? IMO, yes.
