I've had both and I have to say that I prefer my Mark IV combo over my old Triaxis/290 setup. I loved my Triaxis rig. It was extremely versitile, and having the Midi switching was very convenient. However I'd always find myself ending up using only 2 of the modes about 95% of the time, R1 Green and L2 Yellow. I used a TC Electronic G-major with it (awesome unit!), but again I found myself only using a few of the effects in it most of the time. I also grew very tired of the little buttons and menus of rack gear. I was pushing 2 recto 2x12 cabs with it in stereo. It was LOUD!!!! More times then not, too loud. The stereo 2:90 was nice, but most of the time due to limited stage space I'd simply stack the 2 cabs vertically, so the stereo image was pretty much lost. I decided after replacing EIGHT power tubes a couple of times and hauling all of that crap arround for a few years (my back payed for it too), that it was time to downsize.
Now I have a 91' Mark IVa combo (with casters... yeah!) with the EV 200 watt black shadow and a small pedal board with about 5 homemade pedals on it. This rig has all the tones from the Triaxis that I loved, is much cheaper to maintain, and is A LOT easier on my back when traveling. After going back to a knob based interface, I'd have a very hard time going back to the irritating little buttons and menus of rack gear.
Triaxis/2:90 was great and would work wonders if I was a world touring pro, but for small clubs and jamming with buds, I simply find that the Mark IV combo and a small pedalboard fit my needs so much better.
Hope this helps.