It's getting harder and harder to find places where this is true.
ARM Cortex M0/M0+ blows AVR out of the water, and is usually cheaper except for the very lowest end AVR parts. Generally will use less power, too. And that's assuming your unit counts are so high that firmware developer time is free.
Of course, it's getting impossible to find 5V VCC ARM parts, so that's something that would steer you towards AVR if your system is really a bunch simpler by having a 5V micro.
This is not strictly true. Many AVR chips can handle more computation than their clockspeed would suggest due to some really nice assembly codes that allow for common DSP calculations.
I ported an AVR code base to a cortex M4 last year, and some of the inlined asm didn’t translate. I ended up having to use inlined C instead. So, my 120Mhz M4 chip struggled to do what a 90Mhz AVR did no problem.
You can get Attiny to sleep at 6uA with a watchdog, and ~120nA with an external interrupt. Can M0 match that? Genuinely curious, I don't have much experience with ARM.
ARM Cortex M0/M0+ blows AVR out of the water, and is usually cheaper except for the very lowest end AVR parts. Generally will use less power, too. And that's assuming your unit counts are so high that firmware developer time is free.
Of course, it's getting impossible to find 5V VCC ARM parts, so that's something that would steer you towards AVR if your system is really a bunch simpler by having a 5V micro.