Hi.
Anything from Parallax is going to be well supported in documentation and on their forums. And their prices are pretty decent. Also, the SX is stupidly fast (75 MHz with overclocking, one-clock-per-instruction). Lots of people make commercial products with these things, including text-based video generators. And there's even a free BASIC for these, so why hesitate?
However, it's never going to be upgraded beyond its small memory and 28/48 pins. BTW, those 48 pin versions are only available in surface mount flat pack devices. And Parallax is the ONLY place manufacturing, selling and supporting them.
My advice is to consider the more mainstream devices mentioned already, like the PIC and AVR. The most available and commercially supported microprocessor is the venerable 80C51 and its variants.
Of course, it all depends on what you want to do...?
End two cents...
kenjj