It really depends on what you want to do with it, what kind of a rider you are, what you like, what 'fits' you, and what you can afford.
Do you spend time on the freeway? How much?
--heavier bikes fare better on the roads with lots of truck traffic...and typically have more power which is nice when you want to keep up with traffic all day long.
What sort of "dirt" will you be riding in?
--maintained dirt "roads" typically don't require a whole lot of suspension travel and ground clearance. Unmaintained dirt roads need more...single track even more. A KTM 525 is overkill if you are looking to tour the maintained dirt roads where you will probably never see much worse than some wash boards....on the other hand, a BMW 1200GS isn't the best choice for blowing down single track.....
How fast do you ride?
--bikes like the G650GS (or pre 2008 F650GS), KLR, etc are great for most roads/trails if you are not in a hurry. If you are just cruising the back roads and trails (even trails like the TAT), these bikes are great....but if you want to go fast....ehh...not so great.
How much sand, and what type of sand?
--lets face it....heavy bikes typically are not your firend in deep, soft sand. Light is right.....
Do you wrench on your own bikes, or do you let someone else do it?
--if you do your own work, older carburated bikes are easier to work on when things go wrong. But, if you don't do your own work...fuel injection is more reliable.
**I'm going to catch hell about that statement, but I believe it to be true. To those who will argue that carbs are better: Sure, you can rebuild a carb on the side of the trial...but honestly...how often have *you* come across someone sitting on the side of the road because their fuel injection system took a crap? Drop the unsubstantiated, anecdotal story about you buddies uncles friends father in law who heard about this guy who knew some other guy who heard a story at the bar about some guy who remembers hearing a story about a guy that heard someone had trouble with their fuel injection back in 1983.
Besides, it is a moot point anyway, because if you don't have the tools and knowledge of how to fix a carb on the side of the trail...you're walking.
Ergonomics....the bars and pegs can be tweaked on any bike...but if you can't get a foot to the ground fast enough, you'll be picking the bike up a lot more often. Getting that foot to the ground is an individual thing...some guys are completely comfortable on really tall bikes...some are not. Get a bike that YOU are comfortable on. Personally, if I can get the toe of each boot on the ground while sitting on the seat...I'm good.
As with 4x4's...it doesn't make much sense (to me) to spend so much money on the vehicle that you can't afford to go anywhere. A $1,000 ratty old honda that runs well and allows you to afford to go on trips is a whole lot more fun than a $18,000 R1200GSA that forces you to spend every weekend working overtime just to make the payments.
So, the real question is....what do YOU look for in an adventure bike?
But if you really want to know what "I" look for....400+ pounds, 75+ HP, low enough seat that I can get the toe of each boot on the ground (at the same time) when sitting on level ground, water cooled, availability of crash protection/skid plates/etc, enough suspension that I can go for a 'spirited' ride in the dirt (I don't do much single track, motocross, etc), a price tag that I can afford (which changes from year to year). I think I have found a pretty good fit to all of those with the BMW F800GS.
Would other bikes fit? Sure....but the GS is what is in the garage, and I'm really happy with it.
