Engine mileage? Personally, I would say take the XR400. The DR650 is great and all that, but the XR400 is the bike you know. I say this as someone who has a DR650 and loves it. I have spent the better part of 7 months getting this bike ready. Its amazing all the little shit you end up tweaking. I cant tell you the number of times Ive had the entire front wiring portion behind the headlight apart, the number of times Ive taken the carburetor apart tweaking jetting, the number of times etc etc. The XR400 is a reliable bike, its light, and its good for the job. Its prolly also better solo than a DR650. I vote take the XR400, build even more dirt skills, THEN get you a DR and build it up. Use this TAT run to find out what works in the camping and traveling aspect- the bike is the easiest part once all the prep work has been down. FWIW YMMV **DISCLAIMER** I havent done the TAT. I see XR400 and the first thing I think is "Perfect TAT bike"