On topic:
pick whatever floats your boat.
You want board portfolio? Microchip wins.
You want affordable development tools? Microchip wins.
You want good silicon? Atmel wins.
You want good feature set at low price point? Atmel wins (Attiny13 and the like)
You want 16 or 32 bit architecture? Microchip wins
You want DSP? Microchip wins
You want low power? Microchip wins
You want good chip architecture? Atmel wins
You want good community support? They both have it.
You want a beginner-friendly MCU? Atmel wins (just make sure you have a fuse-bit doctor thingie ready).
You want simple assembly? Atmel wins
You want free tools and compiler that are actually usable? Atmel wins.
You want a broad portfolio of peripheral modules (eg. encoder inputs, sdio, hardware parallel port)? Microchip wins
You want to make a single sided board at home? Microchip wins (peripheral pin select in present in some dspics and pic24 rocks)
You are designing a product that will have a long lifetime? Microchip wins.
You want something that will give you a job later? ARM wins. Fatality!