I just pull tested some of my home-made 0.039" wire line clips, they went over 35 pounds of pull without permanently deforming (but were a bit scary to look at). At 45 pounds they permanently deformed but did not pull out -- my courage failed me at that point, and I did not go up to 50 lbs.
This is music wire, which has what McMaster calls "spring back" or "spring temper". "Bend and stay" means "soft, soft, soft". I wouldn't go with anything other than spring temper -- it's the ability of the wire to resist bending that's keeping the whole shebang from unfolding on you, so you want it as stiff as you can. The only place to go from spring temper is "brittle", and that's not what you want.
I've used the music wire clips for over a season in Oregon weather and they're showing no sign of rust. I'm not sure at this point if I'd even try stainless wire -- for one thing, I have something I know works, and for another thing, buying wire from the local hobby shop is a lot more cost-effective than buying it from McMaster-Carr. Further, unless you polish it or get the really expensive ground stuff, it's not necessarily going to be shinier than music wire. But if you do get stainless, get the spring temper stuff.
If you do go with stainless, you probably want to pull test before you fly. For the same temper the stiffness of the wire is going to go up by the cube of the wire diameter -- but the bend radius you'll achieve going around your mandrel will go up with bigger wire, too, and I rather suspect that the pull-test strength decreases with increasing radius. It's all very complicated and my head would hurt if I tried calculating it (and I'd be wrong, to boot). So pull test.
Check Small Parts for alternative sources of wire. Their price per foot is worse, but it comes in longer lengths -- this may make sense for you, or not.
http://www.smallparts.com/stainless-steel-17-7-condition-straightened/dp/B003R5025S/ref=sr_1_5?sr=1-5&qid=1324781473They have annealed wire for a humongously lower price, but then you'd have to heat treat it...