Wide Angle: How picky should I be?

Just an update. I pulled the trigger on a 16-35mm f4, brand new (with the $100 Canon rebate, it seems to be as cheap as the almost-nonexistent used market). I took it out on a quick walk around shoot to put it though it's paces a little bit. It's a fine lens that exhibits all the characteristics described above, and elsewhere on the web: very low CA (even with extreme contrast), sharpness across frame to the corners (although extreme corners forgivably soft at f4), etc.

If I sound underwhelmed, it's only because my impression of this lens so far is that it's greatest strength is that is has few weaknesses. I'm coming from a very cheap, fast, and sharp prime as my main lens, so the fact that I have trouble telling images apart, except for focal length, speaks volumes about both lenses.

I'm still wrestling with the price/value of it for my use. I was just remorse shopping KEH and saw they have EX+ 17-40mm copies for $529-$549, with a warranty. This lens is such a good deal now with the 16-35, that I just don't know if this new lens is worth $550-650 more for my use (variance due to mail in warranties not exactly as good as spot cash in your savings account, IME). Obviously, this is something I must decide for myself. I'll shoot some more and see if I can be swayed and build a relationship.

Either way, I think either of these two is an excellent choice. Again, thank you to all for your thoughts above-- you really gave me things to think about that I couldn't find elsewhere.