If you have a lot of patience, CVS might be the way to go. There are quite a few features in 5.0 that make it worthwhile if you are going to be developing applications with it. Getting it to run with Aolserver 3.x has been quite troublesome for me. Getting Aolserver 4.x running with it was much smoother, although, not without its hitches.
If you are just trying to get the feel for it, and are not worried about an as-yet undeveloped upgrade process, then 4.6.3 might be the way to go. Many of the APIs are very similar and you can get a good feel for working with it without having to wonder if you have hit a bug. The CVS version at times will break (and has).
I have no doubts that the OpenACS team will have an upgrade procedure that works to go from 4.6.3 to 5.0. The OpenACS team is one of the most organized open-source projects I've worked with.
If you're just learning the OpenACS system, I would recommend 4.6.3.
