The 440's are usually fine for trails as long as they are trail converted. Basically, it will need trail heads, oil injection, added cooling, and probably a different clutch setup. If you go with an '04+ you may also want a larger gas tank as well. If you are going to do alot of trail riding with it, I would suggest having the shocks revalved for a bit softer ride. I know my '06 is setup VERY stiff for Sno-X. If you do alot of jumping, I would suggest one.
If you do not want the hassle of converting a 440, and/or you don't go out and beat on your sled (like I do), a standard Firecat would be fine. I'm a fan of the F7's...but I like all the power I can get.

The other's will still be a fun ride, just not as fast on the lakes. I think a Firecat SnoPro would be a good sled, even tho the 440's are a blast to ride!