I've never bought a proper battery for a bike. Ever.

The R75's battery is old, "wet", and not taking charge too well. It's price tag denotes a 1990's purchase.
I'm tending towards an AGM battery, but have close to ZERO experience in batteries. What should I look for? One I looked at this morning - a $200 335CCH/20AH agm seemed a good option, and was slightly slimmer than the wet equivalent. And cheaper. The chap said that the CCH rating was more important. That I'd need all that cranking power. Is he right? The AH rating he said was archaic and irrelevant. Help!