Yeah, thats the thing, it depends on the environment. Just like some bands have come in and wonder why their drums sound like crap in the studio when in their garage they sound awesome...well it's because this isn't the garage. The room is made to bring out all the details of the drums being used...in this case it's showing you that your drums suck. Lol.
The XXX would actually be a better buy IMO if you are trying to save some cash. It's really the same amp. As manny said, the JSX will have a tad less gain and you *can* get more variations on the sounds, but to me those features aren't worth the extra money. The JCM800 2203 is the one that Zakk Wylde uses, well of course modified to his liking, so I would guess that it would be the one to get. I'm guessing (again) that it's the one for heavier music. Don't take my word for it though.
As far as the XXX having more mids than a 5150...umm....heh. The 5150 is notorious for it's mids and well known for it's distinct distortion. The two compliment each other well though thats for sure. The 5150 has one of the smoothest distortions there is, like a...hiss or something, I never can describe it. The XXX has more of a Recto type of sound to it. So where the 5150 leaves off, the XXX will pick up, and vice versa. In a controlled environment such as a studio room made to kill off unwanted frequencies, or with much less bouncing sound waves than a guitar store, an amp will sound much different...then what a mic picks up and you hear in the monitors will be somewhat different than what you hear if you were actually in the room. It's really inevitable. But thats the fun part!
~006