I am unfamiliar with the 50/50, but here's my two cents:
The Triaxis/2:90 are usually paired because you can control the power amp dampening through the Triaxis. Meaning you can switch the modern/deep/half-power settings for any channel you have via the Triaxis, and be able to have different settings on different channels, basically.
I might be a rare example, but MY Triaxis makes a lot of hum on my gain channels. I've sent it into Mesa to have them try to amend that, and though they said they cleaned some stuff and put a couple new 12AX7's in it, it still makes the noise. When I explained this to them, they were baffled and said "all tube amps make some noise" - not like this. This is like having your guitar next to a CRT monitor at all times, and I've tracked everything back to my Triaxis. Clean channels sound GREAT. Gain channels, I have to do a little bit of dancing on my floorboard to mute shit between songs. I'm even using an Ebtech Hum Eliminator in my chain, and going into an NS-2 before the preamp. Here's the kicker - my effects loop makes it worse! I think I have a fucked up Triaxis, which pisses me off because I bought it new and asked about this ever since I got it 4 years ago: nobody knows the answer to this. My tone is good, it's just got an incredible amount of hum to it. I'd like to have my peice of shit G-Major in the effects loop, but I can't even do that.
So...like I said, my end of things might not help your choice, but at least I feel better about getting my problem off my chest.
Are you talking about the Mesa Stereo 2:50? If so, it appears that it doesn't have the same plugin options of the 2:90 as far as controlling it with the Triaxis. I guess your choice would also depend on how much power you think you need as well. In my opinion, the 2:90 is too much for me because I never turn it up past 5(12 o'clock)....even then, it seems like my cabinet flattens that out and there isnt' really any more actually volume to be pushed through it, just more noise/distortion.