Hi Motox415,
I am not much of moto x racer myself, but, do enjoy the occasional track day, and Grand Prix. So, take my opinions with grain of salt I suppose in regards to Moto X. I do think I'm a decent off road racer/rider and technical rider.
I believe the Husaberg to be a more stable platform than the KTM when properly sorted. And way more stable than their Japanese counterparts.
As with any bike, the berg comes with suspension settings that are the best compromise for it's intended purpose. I am a firm believer in having your suspension set up for YOU and your ability/ style. I have found a suspension tuner who IMHO knows his stuff (Steve Searcy @ Clean Racing 714-434-9201) and that has produced a suspension package that is the best I've ever had. And with that comes confidence which is priceless. So, depending on your weight, you might need springs as well as a valving set up more specific to your riding. Your signature doesn't indicate where you are from so no idea about what terrain you are riding in, doring the Hare Scrambles. As with the KTM the ride height adjusment is critical, and is different with the berg, a little less sag I believe, right around 90 to 100 mm, with 30 to 35mm of static sag. I run mine right at 33mm, and that's with 5mm of pre load on the spring. FYI I run a PDS 8, and .44KG springs. I am right at 195 to 200# with all my gear on. This set up is for a relatively fast desert racer, and that means it works good in the high speed stuff, as well as the tight technical, and rocks, we have lot's of rocks out here.
The 550 will have slightly more rotating mass than the 450, and thus more gyroscopic effect, which will make it SLIGHTLY less nimble. The two weigh just about the same. In case you didn't know the 450, 550, & 650 all use the same diameter piston, 100mm. So it's just the stroke that's different between the three.
As with the KTM the Berg comes with an EPA rating so it's lean. The owners manual (which you can download for free from the download sections here, or at Husaberg.se) shows the closed course settings which will richen things up a bit and give a bit more power and make for a broader spread of power. There are seveal threads on this web site of alternative jetting idea's in the fuel seciton of the forums. I suggest you go and have a read. They range from running the stock needle, mj, bigger PJ, to running different manufacturers needles and completely different MJ's. I'm sure you have seen many posts over at KTM talk regarding this subject.
I rode 250 two strokes for years while my friends had all switched over to open two strokes. Once I bought my first berg, a 94 501, I realized what I had been missing out on. Keeping in mind that I have not ridden a 450, or a 650, my suggestion would be to go with the 550, or the all arounder as it is sometimes called. It has great usable power down low, and will really rip in the upper mid range and top end. With the exception of the mass starts associated with desert racing, and really the beginning of any race, I usually use mid range power, just short shift and keep it pullilng. But the top end power is great to have on tap for big sand hills, fast sand washes, starts, and passing people.
I do want to emphasize that the 550 is not too much of handful though. It has very usable and controllable power. It's all up to you and how you twist that right side grip!
Whatever bike you chose I'm sure you'll be very happy with a Husaberg!