According to the Randall Website, the XL cabs are the professional line of cabs and made with 11 ply...the MTS are for the modular stuff and made with the cheaper 7 ply ...my Randall rep told me that the top of the line cabs are the XL cabs.
Why no love for Marshal? Their cabs find themselves on more touring musicians back line than all the other cabs combined. It is true that the back of the newer 1960 cabs are particle board, but that's easy to fix (Mojo music supply)...or just leave them alone, they still sound better than most other 4x12's IMO. but again, I tend to lean towards appreciating the open, airy, dry crunchy tones vs. the woody, compressed, juicy tones. I do think Randall builds a tighter cab that is more compressed and woody sounding...so if thats your preference, the Randall will fit the bill.
But I'll tell you what: I'd put my 1960 Hand Wired Marshall cabinet up against any cab in the world...including Bogner, Orange, Mesa (all of whom I demo'd side by side with the Marshall before I bought it). BTW, the hand-wired cab has a wood back, hand soldered connections and Celestion Heritage series speakers. I bought one BRAND NEW for $950 (these normally retail for a $1200 but you can easily get your dealer to go $1000)...I paid $800 for my Randall XL cab...tell me the Marshall isnt worth an extra $150-$200? Hell, the speakers alone sell for $200 a piece. Like I said, I'll put my hand wired Marshall cab up against any cab in the world and beat it for clarity, punch, build quality, and most importantly, tone.
I'm not sure why Marshall cabs get bad reviews...I avoided them for that reason unitl 6 months ago...now my entire back line is entirely Marshall (a 1960 BV and 1960 BHW). I only use my Randall at home for practice...never gig with it...dont get me wrong the Randall is a fine cab, but for the money, Marshall has better tone IMO...and if you ever try to re-sell your Randall...good luck...I doubt you'll get $300 for it (I've tried to sell mine twice now and cant unload it). The Marshall on the other hand will have a better re-sale value and have a much higher demand if you decide to sell.
Bottom line: I think its a matter or tone preference...if you like a woody, compressed, focused, honky, juicy tone then Randall is the ****. If you like airy, crunchy, open, expansive tones then Marshall is the ticket. Different strokes for different folks...tone is so subjective.