Perhaps the recommendation for AVRs is because there is a free gnu tool chain for them, which is well supported. As for PICs, microchip offers an excellent 'free' C tool set for the 18F parts. (after a couple of months, it won't perform optimizations) They recommend using assembler for the 12f and 16f parts. I would recommend starting in assembler for either PIC or AVR, that way you get forced to know the chip.
Right now, I'm starting into SourceBoost C. I got a full licence for around 50 dollars on eBay, and as far as I've found, it is pretty good. CC5x is also pretty cheap, and some like it. CCS makes some cheap compilers, and they come with lots of built in functions, but I would write my own, so I knew how they worked.
Some people like JAL for PIC which is totally free. It is a bit closer to PASCAL or MODULA than C, IMO. The high end tools for PIC, like highTech's compiler offer a 'free' version, which is code limited. I would try as many as you can, to find the one you like the best. You should take a look at Nigel's tutorials, and the Elmo160 tutorials, just to get an idea.