I feel you are asking difference between AVR 32 bit controller and ARM based 32 bit... Its difficult to answer as both are good in their own terms..
ARM is used mainly as processor... they analyze signals and all that stuff... (Even playing MPEG at little LCDs),but they aren't microcontroller... meaning that they have no good IO capabilities....
An ARM is bad, if you want to replace the 50$ robot... but it's good if you want to make a data analyzer....or do trig functions, or inverse trig functions.... or do decoding....
An AVR.... is good for practical stuff, as getting the measurements, doing PWM, generating various signals.....
But if you want a good answer, then it's only that an AVR is a mule, and a ARM is a brain...
They are totally different things....