I'm in a similar boat, trying to decide whether to keep my '88 R100GS or look for a DR650 set up for adventure rides (like South America etc). I love the R100 (only recently got it so I'm still learning how it works), and any long and dirty trips would be a while from now, but I test rode a DR650 and thought it was a nice little engine (although the bike itself was set up poorly IMO). My 1200GS is about to be hacked (getting a sidecar added) but I think I'd be more inclined to have something simpler for any kind of RTW-esque trips. The airheads continue to do long distances (this one only has 33k miles) and its in my possession, but the idea of the dirt-simple DR is tantalizing.
Grr. Too many danged bikes to choose from!!