If you've already got Reunion...
It's always nice to support good developers and such, but a lot of the new stuff they added to TC is supposed to be what the Xtended mod team had already put into the original. They hired the actual modders for the job.
So realistically, you can already get a lot of the addition from the mod, you'll need to do some research into performance and such to see what it's worth to you, the mod is fucking badass though.
If I didn't need twice the video card I have to run it smoothly, I'd have bought it just to pay them back for Xtended, but the mod alone taxes this system.