Advice, please; I'm driving myself crazy

I'm trying to decide what wide-to-medium lens(es) to buy or keep for my 7D. The otions, some of which I own, some of which I don't, are EF-S 18-55 f/3.5-5.6 IS (kit lens for Rebels), EF 28-70 f/2.8L, EF-S 17-55 f/2.8 IS, EF-S 15-85 f/3.5-5.6 IS.

Below are two pairs of shots taken with my 7D this morning with the most- and least-expensive lenses on the above list. I can't tell the IQ apart, they look equally good to me. Is what I did a stupid way to compare lenses? Can you tell the difference between them? (I'm not going to say which was which for now.) The first pair was shot at ISO 200, one at f/3.5, one at f/5.6; around 50-70mm (and the shorter one cropped to try to roughly match the longer one), center AF point focused on roughly the same part of the scene; the second pair the same, but both at f/8.

FF