The G-flex cabs give off a false sense of bass. In other words, The cab adds to the amps tone instead of projecting what is actually comming out of the amp. I liked the MKIV with the G-flex but you definitely had to dial in the amp differently when using it (Which I do anyway for every cab I own )
If you want a powerfull sounding cab that sounds more like a 4x12 and has a tendency to add to the overall tone , then .. The G-flex would be a good choice. If you want something that is more accurate to what the amp sounds like, The mesa should be fine but I would swap out the speakers (just my preferance.. YMMV)
If you want a 2x12 cab that sounds more like a 4x12 and is more natural sounding ,then I can highly recommend an earcandy Buzzbomb cab.