I have a 1999 600cc Alp and I love it. Totally reliable, rode through the rain that caused the floods back in June and it never missed a beat. (Cruised past a flooded out Range Rover)!
I don't know much about the 650 but Mr Haynes claims 46.5bhp for the 600, 54bhp for the 650. Also, I believe the 650 is geared a bit higher, which may give you a more relaxed cruising speed at less RPM, but I find I would sometimes like a lower 1st gear on my 600 for trickling through traffic or slow stuff off road. I May go for one tooth less on the front come chain & sprocket time.
At the moment, on standard gearing, two up with luggage I cruise comfortably at about 120km/h which leaves plenty in reserve if needed.
Speaking of comfort, this is an area I'm trying to improve on. I may try a gel seat pad or replacement foam and lift the bars a bit with risers. I have a taller screen that helps with the wind blast but it IS noisy! If your riding solo it'll be better, more room to move around!
Fuel consumption on these bikes isn't the greatest either but this is where the 600 beats the 650. Fully loaded, at the above speed, I get 40-45mpg. The 650 can get down into the low 30's, I believe. This is down to emission controls basically strangling the engine but it can be improved with mods to the airbox (easy) and the exhaust sub chamber (less so, but do-able).
I believe the 600 is a bit easier to work on but have no experience of the 650. Rear shock ajustment (preload only) is a pig on mine, no access, which is also true of the forward spark plug on the front cylinder.
Other than that they're great bikes, if a bit porky. Either would be a good choice.
Loads of info here: Transalp.org - Information on The Greatest Multi Purpose Motorcycle in The World.
And here for the 650: Tech Transalp :: Honda XL650V Transalp
Haven't a clue if the engines are interchangeable I'm afraid!
Good luck with whatever you get, let us know!
Hope this helps
Beware the dark, mechanical bird.
Last edited by Crusty; 21 Aug 2007 at 16:08.
Reason: extra word