I think the Atmel designers have made the choice for me. I was leaning towards the 234 anyway because of the EN function that allows me to save power, then I remembered that CANRX1 shares a pin with DAC0 so it's important to be able to tri-state CANRX1 if you are using the DAC.
So it's the 234 for me, no choice unless I add a FET switch or something.
If the library doesn't support controlling the EN pin (it should, we all want to save power :)) then I'll cross that bridge when I come to it.
Rob