In reply to swoe:
I'm not sure which N Wales guide book you have but it's probably safe to assume it has numeric (4c,5a, 5b,5c,6a etc etc) as well as adjectival grades (Very Severe, Hard Very Severe, E1, E2, E3, E4, E5 etc etc.) for each pitch. This might be a good place to start to assess the technical difficulty. It's generally understood that British 5b/5c equates to about French 6a/6a+/6b and so on - add 2 to the Brit grade for the French. Then modify to allow for the "trad" aspects. Some F6a might be only HVS; some could possibly be E2.
I've used these grades as this is about as hard as I've climbed so it's how I understand it. Pretty sure it'll work higher up the grading systems though.
Recommendations? Too many to list. My preference would be, if the weather is kind and the crags are dry, to climb as much as possible on the volcanic rock of the Snowdonia valleys and fells. Whatever happens go to Gogarth for beautiful and dramatic seacliff climbing, but remember it can also provide a wonderful bad weather alternative to the hills, as can Tremadog - great climbing in a very friendly environment. For me the slate would be a last resort but I know some people like it and of course it has many well known test pieces.
Enjoy your trip.