Make "With all the microcontrollers and single board computers on the market, sometimes it’s hard to see all your options. That’s why we made up this quick reference sheet for the 8 most popular boards we sell in the Maker Shed. This handy chart (which we lovingly refer to as the “Grid”) shows the speed, processor, memory, I/O pins, available kits and more to help you make a quick distinction on what board you need for your project. We’ll be keeping this list updated so bookmark it now for future reference! "
Full details at http://www.makershed.com/Articles.asp?ID=302&Click=127972
Strictly speaking this is a comparison of Microcontroller boards, not the actual microcontrollers themsleves, and being Make they don't mention the MicrochipPIC, which is a shame as it has a wide range of devices in hobbyist friendly DIP packages, but when it comes to completed boards it is still pretty much a one horse race for the Arduino, so your real choice is still just which one.