Two things to consider about the 20/20:
1. It has a naturally occuring mid boost (great for lead guitar)
2. You will not get much head room for your clean tone
I owned the 2:90 for a while and it was terrific. It does have the half power switch which supposedly drops the wattage to 45 watts, but I had this switch on for the majority of my usage with it. Even at half power its plenty loud for playing with a drummer, though it will still deliver great tone at very low volumes.
If you're looking for squeaky clean tone at high volumes, the 20/20 will not accommodate. The only reason I'd have chosen the 20/20 is if either I were hell-bent on having easily obtainable power amp saturation at my disposal, or if for some reason I only had space for a 1 space rack unit.
One thing that annoyed me about the 2:90 is its size and weight. I had to buy a whole new rack case that was 18 inches deep to fit the 2:90 in. Then the rack case didn't fit on top of my cabinet so I had to put it on the floor which was annoying.
However if you don't care about your clean tone then the 20/20 might be a good choice. Many have reported that the 20/20 still sounds great for big chunky high-gain playing.