I did the Americas on a KLR650 and it was great. If I had to do it all over again (and I hope I do someday
) I would take a DRZ400. It's the perfect mix of light and powerful. The big CC bikes are great for cruising fast, but often times I found myself having the most fun on small tracks off the beaten path. The KLR was much lighter than the bike bikes, but not quite as nimble as the smaller bikes. Also, it's a pretty simple bike so any mechanic in the world can understand and fix it. So my two cents is for the DRZ400 (just get an aftermarket seat so your butt doesn't go numb!)
REMEMBER: There is no wrong choice and you're bound to have a great adventure however you go! Suerte amigo!!