Switching from PIC to AVR
I work equally well on PIC and AVR (and others, too).

I like AVRs largely because of AVR-libc. It's a decent embedded library that is open source and reasonably well documented (UNLIKE Microchip's plib, but plib does have full source, which I genuinely appreciate). AVRs use regular old gcc and gdb which means no idiotic IDEs getting in my way. The same tools work on Windows, OSX and Linux, and I don't need a 600MB+ IDE with Java and Netbeans or Eclipse to trip over or pollute my source tree with their own garbage.

Programming AVRs is usually done via ISP or JTAG (there was a recent question about that), and from a programming standpoint, it's really not much different from any other micro.

From a personal perspective, I've grown to very much dislike Microchip's datasheets and part numbering. I cut my teeth on embedded design with Microchip and I loved them for a long, long time, but when I look for a micro these days I tend to start with Atmel, although they're not without their flaws too.  (SAM-BA is garbage for their SAM parts, and I've had some trouble with their factory support too.)

As far as which to use... try both, it's like the difference between Coke and Pepsi. They are very, very similar and the choice usually comes down to price and the tools you prefer to use.