I´d say that AVR is better off, as the compiler (avr-gcc) for it is free and unlimited by code size or anything else, with wide range of supporting libraries.
Since there always has been just a single C compiler (now there are more, but mostly not free) the supporting libraries are not shattered by being compatilble only with this compiler and not that one, and the features of the compiler and libraries are well documented and tested.
As for the development board, I would use any of the arduino boards and simply load them with your own code. Again they are well proven. Personally I would go with the Arduino Mega 2560 if you plan on going big, otherwise choose any of the smaller chips that fit your needs.
Or get a TQFP adapter PCB, solder in some of the Xmega series chips, they have even more peripherals available. You don´t need much hardware to support these chips, just a few caps and a power supply.