Hardware wise, most controllers are very similar, and you can pretty much achieve the some results. The cost and programming are two things that tend to set them a part the most. Think about what computer language you are most comfortable with, then see what kind of compilers are availiable and how much they cost. There are free tools availiable on the web, some aren't so good, and you'll realize that you might need to buy anyway.
For high level languages, like C, the internals won't be so important, as the compiler will take care of all the hard redundant stuff for you. In assembly, you should get data sheets and find out what is availiable. I chose AVR ovr PIC, for this reason, and a USB programmer for under $100...