I use PIC, AVR, TI, and am dabbling with the ST32 Discovery.
On customers jobs, I use what they want me to use.
On my projects, I reach for a PIC.
Why?
I really like the community. I like the compilers. I like being able to use Swordfish in 45seconds and have a pattern set, or an LCD functioning.
You REALLY can learn anything from the forums. Here and microchip.com. And thousands of other fan-sites (so-to-speak)
So many schools are using PICs for their base, and quite a few senior projects for these universities use PIC.
This leads to a surplus of HIGH level functions and processes available to us all.
The ATMEL (AVR) gets a lot of its work from the art crowd. The Arduino has a lot to do with this. It helped put uC's into peoples hands that were not even electronics enthusiasts.
If you are an artist and you have a sculpture you want to add leds to, or motion with servos or motors, this makes it a few step process.
I own one.
I have been looking at Bills Junebug, and am expecting one soon. This is an 18f based PIC. This family has FREE compilers available, including Swordfish, a BASIC compiler.
I reach for PIC because places like this, and people like this, have the knowledge and experience to help me out of any trap I get myself into.
Many of my clients have contracts with uC manufacturers and I am required to use that companies uC's if I want the contract.
So, what I have found, If you subtract the "social" part, they are all great devices.
You can grab a book on any of them and do the same things. Processing power, ports, blah, blah, blah...
...They are all competitors of each other and have matching chips to each other.
Some companies have horrible tool chains, and expensive tool chains. That does not sit well with me.
I use linux. I support OPEN projects. I like the fact that microchip researched, developed, funded, and released the PICKits.
This took the big money out of using controllers with REAL power. That is very cool to me.