Re: 5150, 5150 II, or the triple X
It depends on what kind of metal tone you're going for, really.
The 5150 sounds beastly, raw, organic, and grinds like a mother****er. Think Arch Enemy, Carcass, Soilwork, In Flames, God Forbid, Kreator, The Haunted, Strapping Young Lad, etc.
The XXX is more refined... it's like the bastard child of a Rectifier and a 5150, with a Marshall for a nanny. It's tighter, cleaner, smoother. Think Nevermore, Darkest Hour, etc. Not as widely used as the 5150, but just as beastly. It has a MUCH better clean channel, and is definitely more versatile.
I traded my XXX for a the 5150 because I, for the most part, prefer the more raw, organic, crunchy tone out of the 5150, and don't use cleans that much.
I do really miss the XXX, however. If I had the extra cash, I'd definitely buy another, or save up for a JSX. I've been using a ton more cleans lately, though, and probably will be using them about as much as distortion once my G-Major arrives (Heavily effected clean guitars + Tapping = the ultimate atmospheric tool), so I might actually pull a Sikth and replace my 5150 with a JSX. Only time will tell.
In short, you'll be happy as hell with either! Also, Musiciansfriend has the 40W XXX combo's for 500 right now... killer price.