Mark V: little stiffer, more 'modern' sounding. Definitely has it's own sound going on. Three different modes (+3 wattages)mean lots of different tones to choose from. I personally don't prefer it very much.
LSC : Lots of sag, spank and sparkle. a 'throatier' fender sound. Definitely more reminiscent of the classic fenders we all know and love. I personally prefer this amp's clean.
There's so many characteristics In the MkV that may altered throughout the signal path to alter tone, timbre, feel, responsiveness, headroom, character of clipping, voicing, sag, urgency of attack, etc. The clean can pretty well be whatever you want it to be. It's still a Mesa Mark series and it will retain some of its own character regardless of settings. Whether this is a good thing or not is a matter of personal preference. Personally I'm getting some of the best clean tones I've ever used out of the MkV! I'm also a fan of the Lone Star series amps and love the clean tones from both the LSC and LSS. The MkV cleans and OD tones are a better fit me though.