It depends on yourself really.
There is nothing that Smart Thumbs can do, that
TGPX can't do. Sadly, it isn't the other way around. With Smart Thumbs you're limited in what you can and can't do.
So to make it really simple:
TGPX: You are the boss. Everything imaginable is possible. Requires more coding than ST.
Smart Thumbs: The script is your boss, you control the script within it's limits.
I don't want to "bad talk" ST, I have tried both script and this is my very own opinion. Compared to ST, TGPX wins in every single way. ST is more "user friendly" while TGPX gives you plain white pages and template variables and options.
If you want a very easy and user friendly script that requires a little understanding, then go for ST.
If you are willing to take things a bit more complicated, then go for TGPX. TGPX will take some time to get away with. But once you're familiar with it, it's just as easy as ST is. I pulled out my hair when i first installed TGPX. Now however, I would NEVER consider any other script anymore.
Besides, JMB Soft is very well known for their outstanding costumer support. If you decide to go for TGPX, feel free to hit me up and I'll help you out with anything you need

565-013-936