Having ridden Snowdon and Cadair Idris, I'd say that Snowdon is the better of the two, although I've been luckier with the view from the top on Cadair. I remember the climb being 100% rideable until you swing around and go for the final slog. This is the legal way though. At the time, the top section down was some of the gnarliest riding I'd done as I'd only got 60mm forks on the front. Shame the fireroad down the rest of it is a bit dull. I'd say it's well worth a trip up on a bike.
As for it not being proper mountain biking, most of my rides above Vancouver involve walking uphill quite a bit, so if anyone thinks that Thetford is more 'mountain bikey' than the North Shore then you must be tapped in the head!