If it is a mosfet amp, chances are it is linear. That is one good thing about moving away from the BJTs (bipolar transistors). With the MOSFET design it is easier for them to just make it operate at least somewhat linear.
I have seen the older Palomar amplifiers that were stamped AB1 (which is ridiculous because in AB-1 the one refers to the way a *TUBE* is operating not a transistor, so it should read AB if in fact it really is but we won't get into that here) that were actually class C.
I have even seen some that had all of the parts there and in operation for AB mode operation but had the bias grounded out on the input transformers which made the amplifier a class C. They did this on a lot of the black faced, gold faced, and even blue faced Palomar amps because when they built them to design, they did have intentions to make them class AB, but they would do into self oscillation due to no negative feedback, poor RF bypassing and some other issues, so rather than fix the issues, it was easier just to ground out the bias supply through a piece of wire wrapped around a piece of ferrite creating a choke (kind of) and effectively making it class C. In class C you could get away with these issues, because the gain was lowered and you could get away with these design flaws. The correct fix was to undo the class C conversion, add negative feedback, make a few fixes and convert it to AB back to where it was supposed to be.
Not to get on a rant, but I figured I would give some history.
When the MOSFET designs came along, some were good, some OK, and some looked like bad science projects. It's hard to tell what you are getting without knowing what you are looking at...
However, the good news is, that because of the way a MOSFET operates, they kind of (the builders) had to get with the program more often than not, and had to in a lot of cases build a real linear amplifier. Not to say that the linearity is great, or IMD is great from one of these creations, but it's a lot better than what was out there before.
I may be a bit biased (no pun intended) but if it were me, I would say go with a Texas Star 350. I like them because they are mote linear than a lot of what is out there, better components, low impedance source for the bias supply which means better linearity.
But to your original question, the 250 or the 450? .. depends on your drive levels. If you go with the 450 and underdrive it, and let it just cruise along, you won't be pushing the 250 hard and overheat it, distort, go into compression, etc.