Buy them BOTH.
When the DUC is being a Ducati you will always have the KAWI to ride

powerwise i don't think that it would be a problem, but from the feedbac that i got from my wife after her short rides on the back of my mts i wouldn't recomend a multi for any long distance 2-up touring

For 1-up riding you can't go wrong with the 600, for 2-up get the 1000. I have the 600 and never wished I had the 1000.

the 600 is very happy crusing at 85-90 mph all day long

it all depends on your cash situation and who you are trying to impress