The differences include:
625 – no counter balancer
640 – counterbalanced engine
625 – MX gas tank
640 – larger tank (I have both tanks.)
625 – Wired for street legal, but not so.
640 – All the EPA crap & street legal.
625 – Enduro exhaust can.
640 – Portly super quiet and hot can.
The way I see it is that there are no new 640 Dualsport bikes being imported into the US. The 625 is a compromise which allows us to do the bare minimum legalizing needed to get plates. Something KTM cannot do theirselves. Once you add all the street legal stuff to the 625, and remove all the extra crap from the 640, you have about the same bike. Of course the 640 will still weigh a bit more as it has the counter balancer + a couple other things the 625 does not have. I spent some additional $$ changing the exhaust can, getting a Duke gas tank, bash plate and side stand. The charcoal canister just vanished one night – mystery.
For real tight woods the 640 will always be a handful. But for groomed trails, fire roads, and rough blacktop roads the 640 is a BLAST. Ear to ear grins. The darn bike was even fun at Moab
from
http://www.dualsportmagazine.com/Moab2003.html