Hey Andrew, And welcome to Umbraco...
If you want to run the sites in production, you should use v3!
If you just want to learn Umbraco, and play a little around with it - you should choose v4 beta1...
It wan't be a problem to you, if you install v3 now and update to v4 RC1 (or the final release) when they are released.
You should check the milestones for v4 development - it will give you an idea of, when you could start using v4 for real!
You find the milestones here:
http://tinyurl.com/6m5k7xHope this post helped you :)
Kasper
Work:
pakhus14.com Personal blog:
pepfugl.dk