I've been to CMAC, Darien Lake, the Amp and SPAC.
I admit that I haven't been to the Amp since last summer's Phish show, but that was a dreadful experience. The Dave Matthews crowd at this summer's show was also an awful mess, although they got the parking lot done a couple weeks before the Fair, but it's been a problem most of the summer.
I went to SPAC for Evanescence with the orchestra this summer - about my third time there. I had seats under the roof each time. SPAC's seated area seems larger than the Amp to me, maybe because it's at more of an angle, and maybe that makes it seem bigger. The Amp is a more gentle slope. I agree that SPAC has insufficient room on the lawn. No question.
But there are 2 ways to drive into the park where SPAC is located, from the Route 9 side or from the Route 50 side. If you use the back way in and out of the park, the traffic is much easier. It's actually harder walking out across those 2 small foot bridges within the park that make leaving the place so tedious; at least that's been my experience.
Darien Lake was easy-peasy. CMAC depends on the show, I guess.