I'd go medium, but you could use either. The Saints have strong springs anyway, but medium-cages tend to be a little stiffer. I assume you're running a chain guide, too, so maybe that's a moot point, but the stiffer springs do help a tiny bit with chainslap on the chatter and on guide-less bikes, with throwing the chain. You'll also gain a teeny bit of clearance and lose a teeny bit of weight by removing chain links.

With a good FR derailleur like Saint/Hone, it won't matter a whole lot, so you can pick either one. I've been losing the big ring from most of my bikes and switching over to medium-cage and like it a lot, but it's not a huge difference really.