Hi Mark,
Glad to hear you've seen the light....! To be honest, either of these bikes would be fine. I've had at least one C15 since I was 17, and I've also owned AJS-Matchless 250cc/350cc models too. Spares for BSA's are probably more widely available, but there isn't a problem with AMC spares either. Both are easy to work on and relatively reliable - providing they are ridden within their limits.....! I think it will be down to the bike that catches your eye the most...
If you're interested I'll be selling my Matchless G2 250cc very shortly (just about to get it MoTd). Email me or drop me a message if you are.
Nigel.