I think, in very few lines, that Jmaster refers to the textures each units/building has in both games (correct me if wrong). As Moeller and Pop said from the very first posts, the demo might has limited texture details. I don't know if this is true, i would add the word might have, just to be within the odds. Besides, switch all graphic rosters to full (antialising especially) and you'll see very good images (i've posted 2 of them). Yet, i have not seen an Anisotropic filter roster in the option, and since Antialising and Anisotropic filter go together in the latest games, Moeller and Pop might be right.
Anyway, SWGB was 2D as mentioned. It's always easier to model a 2D model than a 3D; all 2D models have 6 possible sides -so, load them with as many textures as you can and you're done. A 3D model has all aspects that must be modeled and so, it might take more time to model it as you should with a 2D one, but this doesn't mean that the 3D model will be less detailed than the 2D.
Now, i think the buildings in SWGB had some lights, windows, doors, and they looked "heavily detailed". In EaW, buildings look a bit more simplified. In this way, SWGB did had more detailed buldings, but mind that they buldings had only one aspect, so, what "thanks a lot for the textures".
I don't think this applies to the units, where EaW has way more good-looking and actually textured ("good-looking" is a very biased word) units; both ground and space. Just zoom in. I think, for ground units, Force Commander can go toe-to-toe with EaW, taking of course in account when it was made. In few words, FC has about the same modeled units (they don't have the same number of course, im referring to the common ones), but due to the technological disadvantages of that time, the units where too "polygon-looking" (i think antialising was not a broad tech by then), which was obvious if you zoomed in.
Yet, do not forget that we played a demo, and as moeller and pop said, the final game may surprise you, if you can play it at max graphic specs.