When it comes to "being used to"...it makes no difference. A bike is a bike. You will find just as many "little" differences between your '12 YZ as you would going Orange.
Getting the KTM, none of your spares will fit. Getting the Yamaha, only 1/2 your spares will fit - and probably not the ones you want.
Yamaha has better suspension but it has an Aluminum frame. (you all know my dislike for aluminum frames)
Here is what I found was the most important issue when considering a new bike: Dealer Support.
If you are buddies with your local dealer, get a 'fair' deal there, they stock or can get your parts quickly, are knowledgable on your model and in your sport - buy that bike. If you are 200 miles from the nearest dealer, and he is a jerk, and you need a part - you are going to miss the race and then blame the bike manufacturer for not anticipating this problem.
I want a dealer who races. I want a dealer I can call on Friday night and say "I need 'x' part, can you bring it to the races Sunday?" Then not have to worry about the bill until Tuesday. Or even better, one that anticipates problems..."Johnny, I got a flat, you got a Tube?" "Sure Kurt." or "Steve, I grabbed the wrong spark plugs, the ones for my Penton. You got a plug for the KTM?" and I get the answer..."Sure Kurt."
The differences between a Yamaha, KTM, Husqvarna, Suzuki, TM, or Beta are so small and unimportant that it just comes down to support and choice of color. Oh and one more thing...
...With the KTM, you'll need new riding gear.