Ergonomics, price, simplicity of design, repair parts availability, track record as far as reliability is concerned and availability of holster and repair parts would be my concerns if I was looking for a primary defense side arm. However, if you already have a other handguns, it really is down to which one gives you the warm and fuzzies when you pick it up.
I like Glocks, H&Ks, CZ's, Browning, Walthers, oh and Sigs, Steyrs oh my... If I were considering a medium framed 9mm, I'd get both. And no, I am not married...my gun habit is not a negotiation point. When and if H&K comes out with a .45ACP version of the VP series, I will be adding that to my list.