I plugged into my Express 5:50 combo last night and it sounded great. I then plugged into the Mark 5 and that sounded great as well. The difference though is fairly decent. I've played with both of them together a while back when I first got my Mark 5 and I was able to get Express tones out of my Mark 5. However, you kinda have to work the Mark 5 to find the Express tones in there, while you can just plug in the Express and get the Express tones without needing to tweak anything.
I kinda like having an amp where I can just plug and play and get great rock tones without needing to tweak anything...I love my Express 5:50. And yep, it is a very midrangey amp. I like cranking my mids all the way, keeping the bass at 9 o clock, the treble at 12 to 2 o clock, and the contour control full on. I find it stays fat in the clean channel, and tight in the burn channel. Actually this sounds great whether the contour control is on or off. Although, the contour control always blows me off my feet when it's on.
Anyways, the MARK 5 is a great amp too. It can get Express tones and it does a lot more as well, but the Express is a lot easier to dial in than the Mark 5. I highly recommend the head though. Those combos weigh a whole lot. I have a Mark 5 head and it's very portable. I bought the Mark 5 head and a 2x12 Recto cabinet and that goes great together.