I have a 75g tank myself that I plan to set up as a display for a false water cobra one day. But I certainly would NOT recommend one of those for a first-timer though.

The 55g would make a fantastic dart frog setup! Or a small (2-3) collony of crested geckos would be nice in that too.

A rainbow boa would be great for the 75g. Also a rough green vine snake would make a nice display.