Check Point's got some docs on the differences. Start here maybe:
http://www.checkpoint.com/ngx/upgrad...new/index.html For basic setups, the main differences you notice are in the GUI - lots of nice improvements for basic tasks. Cloning objects is a massive timesaver if you're creating many similar objects. Tooltips showing the IP address of objects. Ability to look at an object detail while looking inside a group. Smart Defense profiles.
There's also support issues intertwined with it - FP3 goes out of support in about 6 months I think. If you ever get the newer Nokia hardware, you need to run the newer IPSO versions, which need newer CP code...
Personally I would go for R62, as that is the most recent release, and incorporates fixes in R60 up to HFA 04, and R61 to HFA 01.