Comparing a ktm 250 sx with a honda 650

OMG, Compare a greyhound to a St. Bernard.

The 650 is good for open riding, and big guys can lug them around O.K., but I can't imagine racing one (other than Baja).

The 250 SX is a REAL machine. It is probably within 5hp of the 650, weighs 75 lbs less, and turns on a dime. If you get one I'll bet you will be intimidated at first. That bike has some REAL POWER.

The 125 SX on the other hand is a terrific bike, but has that 125 2-stroke type of power band. It is a TERRIFIC MX & SX bike (it just finished 1 &2 for the season in U.S. Motocross). It is a little tougher to manage in an enduro though.

Delta Burk vs Christa Flockhart.

The SX is a great dirt bike
The XR is a large dirt bike

Mazeco, Four- strokes are not inherently better than two-strokes they both have advantages and disadvantages. It really depends on the track. Big, wide-open tracks tend to favor the 4s, while tight sx style tracks usually favor the 2s.
Reed has switched to a 4s for the rest of the nationals, and Carmichael has mentioned that he may switch to a 4s for next years nationals too. However, I believe that they will both be on 2strokes for sx. The day may come, if 2strokes survive the initial 4s onslaught and the EPA, that riders will ride either a 2s or 4s depending on the track.
I have owned both and I prefer a 2s. For me, 2strokes are just more fun to ride, and they are what I grew up riding. Since you are used to riding 4strokes, you may prefer to stay with a 4s.
If possible, try to get a ride on a 2s and one of the modern high-performance 4strokes and see which you prefer. Either one would be a major departure from what you are used to.
Good luck.