Hello...a couple of things:
1. Will your structure be attached to your house?
2. Will you be heating/growing in it year round?
Most growers will tell you to get the biggest size house that you can afford--you will quickly fill it.

As far as year round growing, glass GHs are cool to look at, but are inefficient to heat, and can be dangerous in areas with snow load/hail/high winds...ultimately it depends on what your primary purpose for getting the gh is...