What year KTM are you coming off?? You know the best person to answer this is Mr Clay as he's had ample experience with both bikes.
The major differences noted here are that Gas Gas has spent the last 14 years addressing issues and refining their product. There hasn't been any major changes over the years but all the minor changes improve on the last and make a great bike.
IMO the major difference you will notice between the KTM and the Gasser is how they ride. The way they feel is completely different. The Gassers (especially 2011 and older) feel very japanese in the Ergo department. You really sit on the bike and feel like you're sitting in it. For me the KTM always felt like I was just sitting on top of a rocket ship. The Gasser will bring back your confident in a bikes ability to handle. You won't add a steering damper because you have to, you might add one as a safety feature. You'll be blown away how balanced the chasis is and at the bikes ability to change direction. You'll find yourself dropping it up the inside line more than you ever have. They weigh a bit on scales (which everyone brings up), but when riding they are much narrower than the other bikes and feel really nimble. The weight is only noticed when lifting.
Maintenance. The older the bike the more qwerks you'll find, mostly regarding the fittings and fixtures. My 2010 goes together really well, as long as you have every imaginable size socket, allen key, torx bit, spanner. Haha! Its not really as bad as I make out, but its definitely not all 8,10,12mm like it should be. The 2012 is said to be much improved. Keep the bearings greased, fresh oil in the tranny and the air filter clean and you'll see one of the most resiliant engines out there. I pulled my first piston at 75hrs and still had 0.5mm ring gaps. GMP recently rode his 2012 with no coolant (thread of its own) and didn't result in any damage.
Engine wise. Its just a 2 stroke. Stock they come pretty average with some big tolerances to ensure the can run on fuels world wide. Take the time to check the preload on the power valve, and if you don't mind putting a few dollars in get the squish set. Best value mod you can do and will make the bike more resiliant to changes in temp and altitude. You'll pick up some grunt top to bottom and improved milage too. The PV is a very basic design that works. The engine responds very well to jetting and can be tuned to run like an MXer to silky smooth just by changing the needles around (basically same as any 2T). The engine doesn't feel all that powerful or explosive, but it is deceptively quick.
Suspension is a tricky topic as it involves the person and their riding area and style a lot. Some bikes came with 48mm CC Zokes, 48mm Sachs OC Forks, or 45mm OC Zokes. The CC are the pick if you can get a set. They are only on 2011 Race models and 2012 models. The OC forks can work well with a revalve but some people have struggled to get them to work well across a variety of conditions. A limitation of their design more than anything else. The shocks are either Sachs, Ohlins 888, or Ohlins TTX. All good and all great after a revalve. TTX inserts are also available for all forks if you feel you want to bling it! Out of the crate the 2011 Race or 2012 models come best to just ride without any work. They are the pick! Re spring for your weight and you're gold.
They are botique, but they are moving forward. The last few years I have seen more people buying and asking about them. At least here in Australia they are pretty popular. Parts availability is a non issue for consumables. Some rarer parts can take some time to come from the factory, but thats the same for other brands too.
I think you'd be mad not to consider the Gasser, but at the end of the day its only you that will know if you want one or not. Sit on one and see how it feels. If you get to take one for a ride you'll know why we all own them.
I have rode a 2011 TE 300 (blue ktm with CC forks) and it was nice. The owner of that commented that the Gasser (2010 EC250) felt a bit stiff, but turned well, went exactly where you pointed it, and wasn't short on power.
I honestly haven't ridden a 2012 Ktm so can't compare to that, but I can tell you I have never heard of anyone changing the triple clamp offset on a Gas Gas to try and make it steer/handle. Just sayin
The choice is all yours!
Cheers
Jake
and launch off the trail I can reel it back in and get back on the trail where when I would get in that position on my 200 I couldn't save it and would sample some tree bark. 
