I put a Motobatt battery in my curvy 18 months ago and it's as good as new, even after a month in the cold. It's also got 2 sets of terminals on it making accessory attachment neater - bike electrics on one set of terminals, Bibio's relay'd splitter box thingy on the other. And, apparently, Motobatt customer service is the mutt's nuts, not that I've had experience.
But in any case, get any branded AGM (absorbed glass matt) battery. But DON'T LEAVE THEM ON TRICKLE CHARGE!!
EDIT: Here's the "Texas Toast Battery Cook Off", worth a look. The "Texas Toast Tombstone Turnover Test" (lol) appears to be the most relevant to hardcore cranking ability,
Linky. Motobatt wins hands down.