Generally fish tanks seem to be more widely available in certain places. Here in the UK most people I've met use the wooden vivariums, which is what I personally use. There is not a huge difference between the two. The only things I can think of are glass tanks have more ventilation (which can be a pro or a con) they are easier to get into to clean and they may be the cheaper option of the two. The lighting and heating all sits on top, rather than inside the cage like the wood. Wood is a better insulator, however setting up the lighting inside the vivarium can be a little tricky if you're not into a little DIY. The side doors make it harder to clean the tank, but easier to insert an remove things in there I believe, as you don't have to take lighting and heating units off the screen to get into it.
These things don't make a huge difference though, but you're best asking some one who has used both
Size wise the minimum people use around here is about 3x2x2. 4x2x2 provides a better heat gradient. 5 or 6 would probably use more electricity to heat the whole tank to the right temperatures. They would certainly use the extra space though.
Hope this helps a little
