I've noticed lately that the differences between DSPs and MCUs are tending to disappear. I use Motorola DSPs (56800 series) and they are as simple to use and program as an MCU.
The real power comes when you optimize your software for signal processing, especially if you program in assembly language.
I must say that regarding programming in C, it is almost the same to program both. Regarding the tasks they perform, they are used in different environments: DSP for fast signal processing, and MCUs for control and other applications.
Mariano