One of the reasons that 8-bit AVRs are still popular is that the architecture was specially designed for C - compared to many other popular 8-bit microcontrollers, it has a C-style stack, lots of registers, and low latency for most instructions. The 8-bit PICs are much less amenable to C compilers.

I also use the ST micros for most of my projects, but the AVR is also a great device with a well supported toolchain - I think it's generally easier for most beginners due to easy to use peripherals and simpler clock trees than the ST chips, while still being production-ready.