My $.02…. You need to compare two specific 1x12s to one specific 2x12 all using the same speakers. There are quite a few variables involved in cabinets that change the sound considerably, so much so that generalities are not going to be very reliable or helpful.
That said, I’ll give you some generalizations anyway…. I’m more likely to like the sound and feel of a cab onstage the bigger it is. The smaller it is, the less likely I’ll dig it. There are exceptions to both though which is why you need to try it and compare things directly. Smaller boxes usually sound smaller IME, YMMV.
Also need to consider what you’re using it for and what matters most to you. If it’s mainly for sound/feel and I don’t care about logistics, I’m going with a big cab. If I’ve got to carry stuff around much and ease of use is the thing that matters most, I want the smallest, lightest thing I can get away with. My preferences for larger cabs go out the window if I’ve got to carry much stuff around or I have limited help.
Is this just for stage monitoring and feel, with FOH taking care of virtually everything the audience will hear? If so, you can get away with a lot. If you actually need the audience to hear your cabs, that changes things a lot with regard to cab choices. My oversized Mesa 412 was glorious if you were standing in its path, get 1* out of its path though and you weren’t gonna hear the guitar at all. If I need it to be widely dispersed I’d be looking for multiple smaller cabs and maybe even open or semi open backed ones. I don’t see many gigs like that anymore, but if you have one it is something to be aware of.
D