IIRC, the XMEGAs have a lot more going on including an event system, stacked interrupts, some form of DMA (?), a real onboard DAC, higher res ADC... Sounds like a splendid chip, I'd read in the old arduino forum it was hyped up by Atmel but they fell short on producing the chips so nobody could get them (QC issues? plus an actual bug in the DAC output?), I guess that's all overwith now but ARM is eating the lunch of anyone looking for anything >8bit IMO...