I'm going to have to upgrade my case soon, and I'm torn between these three. I'm mainly looking for maximum air flow, but I want it to look halfway decent too, not just a box with mesh... though if that's what will give me the best flow, then so be it. Also, nothing huge; just a standard mid-tower ATX that will fit either ATX or M-ATX boards.
I would prefer a side panel window, yes, but if the air flow is really that much better with the Challenger compared to the other two, then that would probably be for the best.
However, what made you choose the Guardian 921 over the LEXA S? The LEXA S has both 1 extra top fan spot, as well as 1 140 mm fan that the 921 doesn't.
Also, I forgot to mention that it should (hopefully) be roomy enough to fit an EVGA GTX 660 ti, for those that have had experience with it. (Going back to the mid-ATX vs. full-ATX case, if that becomes an issue.)