Forum Moderators: phranque
Assuming you mean Apache 1.x to Apache 2.x comparisons, one of the best places to start is the official documentation ...
New features with Apache 2.0 [httpd.apache.org]
Upgrading to 2.0 from 1.3 [httpd.apache.org]
I'd go with 2.2 if you have that option. Otherwise, 2.0.x is close enough.
There could be compatability issues with specific packages or OSs. Obviously, this could limit your choices.
It's a practical matter of going with what is current, widely-supported, and you are able to grow with in the future.