I've never owned any of those, but I've heard-

-The XXX is more harsh sounding than the 5150
-Do you mean the 5150 or the 5150v.2? The clean channel is different, I believe.
-The triple rectif. isn't really any better than the double or single
-As far as wattage goes headroom can be (I've never owned that amp so I can't say for sure either way) a consideration, although there are several other threads over the issue of wattage here with some very good points on both sides and way more knowledge than I have. I highly recommend you do a search and read them sometime, it's a very interesting subject.

Some further thoughts-
-Have you looked at Engl? They make a great line of amps which I believe have the american distortion sound it sounds like you're looking for, unless I'm horribly confused from it being early AM.
-Which of those amps have you had the opportunity to play around with using your gear? It's very important to know exactly what you're getting when you buy an amp, especially the relatively expensive ones like Mesa. We can't tell you what you're going to like, just give recommendations.