More people play 1.1. Enough play 1.2 to be reasonably sure of a well populated server when you feel like playing though.
Yes, there is a default gpp build for mac which works pretty well. If you want to try 1.1 try the mac build of tremfusion. Also try segfault (search the forums) for an alternative gpp client, which kiwi compiled for mac.
IMO, gpp is a lot more fun/dynamic/interesting/skillful/logical, especially building wise. Clearly people disagree on this though, and quite strongly, as can be seen from the use of large red fonts in this thread.
Check out the threads in the developer's corner section if you'd like to see the exact differences. They are pretty much all gameplay related variable changes, though there were a few graphical improvements too.