I don't have direct experience with these particular cabs, but from my experience with other Mesa cabs, the 23" will sound boxy in comparison to the 27".
I do have firsthand experience with a 5:25 combo and a Mark IV combo, with and without a 3/4 Back 1x12" cab. The 3/4 Back cab has a very full sound, and the 12" speaker just sounds huge. Nobody ever thinks it's just a 1x12".
So back to your question - I'd get the 27" cab, or if you could live without a matching cab, I'd seriously try the 3/4 Back.
Full disclosure: Yes, I have a 3/4 Back cab in the classified section, but I've been recommending this cab since long before I was attempting to sell it.