Damn, I think most of the other guys in here have traded their ATKs in on 525s. I guess it's up to me to chime in.
My current garage includes two 1997 ATK 605s and three KTM LC4s. So I can give you an honest appraisal of both types.
Is the ATK an adventure bike? Not a chance. The subframe is meant to keep your butt off the tire in between rough sections. It's a spindly aluminum thing that won't support any kind of real hard luggage. You'd get away with lightly packed soft bags but that's it.
The previously mentioned fuel tank will limit you somewhat. It's not a deal-breaker for me but it does limit your range to about 100 miles in slow-going or sand.
The early model steel swingarm has been mentioned but then the very late aluminum swingarms were rumored to be worse. One of my bikes has cracked its swingarm, the other (that was ridden harder) hasn't. Ironically, the bike with the good swingarm has a crack right on the weld at the back of the steering head (infant mortality me thinks). I don't think their welders were the best in the world and that may have contributed to the inconsistent quality we see.
The exhausts came in two flavors: 1996-1997 and 1998-2003. The early exhaust uses a single bolt to hold the muffler up. That style will fail eventually. The good news is that you can buy the whole later exhaust and a weld-on bracket to update the older subframe. Some bikes already have it. Depending on how hard you ride, you may or may not need it.
The problem stems from the heavy muffler being unsupported and placing all the vertical loads onto the inlet flange. It doesn't last long. If you buy an early model and catch it before the muffler fails, you could fabricate a second hanger on the muffler and prevent it from happening in the first place.
I too chose the Rotax engine because it was aircooled and meant never being stranded with a leaking cooling system. Ironically, I got the ATK super hot while plodding up a 10K foot 4x4 trail in the Sierras last summer. The loctite let go on the belt tensioner nut. The belt came loose, skipped a few teeth on the cam and bounced an intake valve. Luckily it happend on a public road and I was able to get a AAA tow back to camp. Five miles earlier and I would have been really screwed.
As for road riding, the ATK sucks. It's a dirt bike. The lack of a cush drive hub makes life harder on the transmission. The flex of knobbies will cushion the trans but if you go for a dual-sport tire like a Metzeler Sahara, it will hammer that poor transmission.
The KTM 640 Adventure has a steel subframe (can handle luggage), equal suspension and more than double the fuel. They are both great bikes but the ATK is a brute of a dual sport and the 640 Adventure is an adventure bike. Even with my oversize headlight and Acerbis aux tank, the ATK is 75 pounds lighter than my KTM Adventure. There's a reason I have both. If I could only have one and I wanted an adventure bike, there's no question: I'd keep the KTM.