Both are good for easier problems, although personally i think RAC, while being a bit more limited, is a lot nicer. I find the climbing on the Cromlech boulders a little bit grim, and it's also a lot busier and right beside a busy road. Both have really nice views.
Either way you can't go wrong. The guide that you mention is the one to get, and it's got all the info you will need. If you're down that way go and ask in one of the climbing shops in Llanberis and you'll get good advice.
No, I don't think you do have to knock on any doors for the RAC boulders.