Both Keil and Tasking have excellent compiler/debugger suites, as do a lot of other compiler companies; however, the kicker for me has always been Who Returns My Phone Calls? Some compiler companies have great sales teams and terrific technology but non-existant technical support.
Keil, for one, has always answered my phone calls & emails, and has always resolved issues to my satisfaction.

I can't say the same for many of the other compiler companies out there, whose idea of tech support is a telephone hooked up to an answering machine.

-Bill Giovino
http://www.microcontroller.com/