I just switched to an nvidia gtx 580 from 3 amd 5850s. The drivers for multi-gpu users are terrible on the amd side. The grass is not greener over there.
When I was comparing different alternatives, I know it sounds funny, but I listed framerates for the resolution I play at (or the next one up if that wasn't available) from the various review sites for different cards in a spreadsheet. It makes it easier to compare. You can start to average things out that way. You should also note the different features on and off have quite an impact on framerates (AA, AF, HBOA, etc...).
More than that, though, for me the AMD driver releases for the last few months gave me one issue or another with my crossfire setup. To me, I had way too much of an investment in the technology to have to deal with the monthly issues with the drivers. It seems you're better off as a single-card person with AMD. That's been my experience. All my single card AMD setups had no driver issues.