Widebears right the GasGas MC250 is a great off-road bike, especially if you find yourself riding both MX tracks and doing off-road harescrambles /enduro. The six speed transmission alone gives you more selectivity then the five speed KTM XC transmission.
When you factor in the heavier sprung 50mm forks and shock (normally spot on for a 200+ lb off-road rider) found on the MC250 you have a suspension that's awe inspiring, espcially if you come up short on a 90 foot double.
We take the MC250 to the next level be swapping out a 300 top-end to broaden the motor’s power delivery and tame it down a little with flywheel weight that smoothes out the bottom end.
The MC300 motor isn't any faster then a stock MC250 it just has more torque, closer to what a 450 four-stroke bike has. The biggest reason for going with the MC chassis is it has the 50mm Marzocchi forks over the 45mm enduro ones (the EC chassis.)
Speaking of 450's if you want a bike that will run with the 450 four-strokes on the MX track one weekend and dice through the woods the next I would recommend looking into building a MC300.
It makes a lot of sense especially when you factor in the two-stroke reliability and its low cost maintenance compared to the PIA four-stroke’s with their expensive top ends and valve maintenance.
As far as a head to head comparison between GasGas and KTM two-strokes go, traditionally GasGas chassis and suspensions need less setup, arguably GasGas engine reliability is a little better (remember this is a GasGas forum website so your not going to find much KTM bias here) while two-stroke parts availability is about the same. Both bikes use a Kokison dual map ignition.
As far as the clutch goes a GasGas clutch is probably the best off-road clutch available hands down while the KTM clutch needs a $50 roller bearing lever to ease up the activation a little, but even after this they still fall short on the feel that a stock GasGas clutch has.
KTM resale is definately better, especially if you only keep a bike for one year before selling or trading. The longer you keep the GasGas the better you come out on resale, keep in mind the GasGas motor is built to last and you can ask anybody around here about engine dependability.
KTM aftermarket parts are also more readily available as it seems everybody makes something for the Austrian motorcycles while GasGas aftermarket parts are a little limited.
IMO if you want a solid all-around bike, with great suspension and a six-speed transmission go GasGas. If you don't mind spending between $400 and $700 on getting your suspension right and want higher resale value go KTM. In the end the you end up spending about the same.
Sorry this got a little long.