Truespace is a much better modeling program over gamespace mainly because it is expandable through plugin packs, gamespace is not. Gamepack turns truespace into gamespace as well, so exporting texture and different .x formats becomes avalible. The depack, raypack, antipack, fxpack, level editor add different tools for slicing, welding and manipulating your models making levels, along with more controls for particles and animations.
Truespace also has a build in physics for animations, say if you wanted to make a house get destroyed by a tornado you could apply glass, wood, paper, iron style physics handelers to the seperate objects that make up your house then make a wind emitter and just click the simulation button and the engine would create your animation using physics. Also has a build in facial animator that supports text to speach and .wav support (not sure but gamespace may also have this).
If you plan on doing any type of cutsceens in your games or want to create movies id go for truespace over gamespace as it is a more complete modeling package more along the lines of max or maya although not as good as either but a lot cheaper. If you just looking for a modeling package for game characters id go with gamespace since it is a lot cheaper then truespace esp with the all the different packs since they run around $200.
I would suggest you might try both programs demos and read up on the features both offer, think about what you are going to need out of the program now and in the future before your purchase either. It is better to buy one program that will suit all of your needs instead of getting one based on price and be hunting for other programs later down the road as you advance in what you are doing and what you will need the program to do.
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