I did a comparison between Zynq-7010 (XC7Z010-1) and Cyclone V SoC (5CSEMA5F31C6N).
The Fmax is 380MHz for the Zynq (should be identical to Artix) and 200MHz on the Cyclone V (fastest speed grade). In my several years of experience working with both Altera and Xilinx parts it was consistent that Altera parts are slower, but only recently did I directly compare and get some numbers.
I'm a lot more comfortable with the Altrera tool set/workflow/IP these days making a switch to Xilinx difficult.
This is often a very good reason to choose a device -- you already know the tools, so the time to get the design completed should be faster. And if you're not pushing the performance envelope, then whether the devices are made by Brand X or Brand A (errr, I) or Brand L or brand M might not matter. All things being equal, of course.
If the design requires a feature that one vendor has and the others do not, you choose the part you need and if you have to learn the toolset, you do. But we're professionals, right? Learning a new toolset shouldn't take very long.
If your production quantities are such that per-part cost is significant, then you choose the cheaper part and learn its tools.
Of course, learning the tools is one thing. Switching to a new device family, even from the same vendor, will require learning the details of that family and how to exploit the features.