out growing 8-bit AVRs, not sure where to move on to
ARM, ARM, ARM.

ARM license their processor cores to many companies. This means that you'll find good tools, support and documentation from more than one source.

PIC, AVR and MSP430 all suffer from the problem of being wholly owned by one company.

Bear in mind with ARM microcontrollers that a Cortex-M3 from NXP is going to be closer to a Cortex-M3 from ST or Luminary than an ARM9 or ARM7TDMI from NXP. More often than not, compilers, debuggers and programmers are common across cores rather than manufacturers.

Getting the Codesourcery GCC ARM toolchain and a cheap ARM JTAG dongle will get you a long way.