Well, that depends on the handgun, what yer gonna use it for, etc.
If it's a collector gun and you're not going to shoot it, don't bother. Unless you can get extras cheap, skip 'em.
If it's a shooter, is 11+ rounds really necessary at the range? If not, get a bunch of postban tens and save the money for ammo. If you do want more, then get a conglamoration of tens and ten-plusses. Save the plusses, as if they break it's a co$tly loss. (Imagine a $100+ pistol high cap falling to the ground, loaded, and hitting cement or sharp rocks. Now, imagine the same scenario with a postban ten.)
If it's a ccw gun, you've got some choices. Most gunbattles [b]don't[/b] involve more than 3-4 rounds at best; ten should be fine. But, Murphy's Law does take effect at bad times; more is not a bad thing. Still, carrying stuff with you can get heavy and there's that breakage issue. In this case, have a conglamoration again; a ten in the gun and a ten plus in the pocket...
Personally, I bought five Kimber high capacity magazines (Kimber Polymer series) and I don't even have the Kimber yet! And, I want MORE.
I'm a mag-o-holic; I'd prefer ten. But, I can settle for six. :D
Mike
PS. If possible, get stainless mags. Less worry about rust.