I have both in first generation models. I like both and use both, together and individually. I play in two moderate volume bands plus a church band. In the 1st gen, I would choose the 12-3. It just gives more richness and fullness in the bottom than the 10-2. The 10-2 is not lacking and I use mine both individually and in a pair, but of those two, just on sound, I would pick the 12-3. It would clearly be more than you need (as would the 10-2 for just playing at home). All that said, everything I've read sounds like the 2nd generation 10-2 has more of everything than the 1st generation one had... so that may change the calculus.
As has been said, the 10-2 in either generation punches way above its weight class. It is adequate on its own for many gigs and does not sound like the tiny cab it is. It does weigh less and has a smaller footprint, both characteristics could be important for your use. The first generation 10-2 would certainly meet your needs and more and the 2nd generation would be more so. The 12-3 1st gen is a killer cab while the 2nd gen is reported to be better yet. And the 2nd gen cabs weigh less. Bottom line is that any of those choices would be wonderful.
I toy with the idea of adding a 2nd gen cab to my rig for lower weight and improved sonics and like you, go back and forth between the two models. Thus far, the deciding factor has been that I really don't need another cab... and if I were to make a purchase, looking at some of the super light weight cabs that are now available may make more sense for my old back. I've heard the GR superlights and have been really impressed with them. Very punchy and aggressive sounding cabs... I haven't heard the lightweight Markbass cabs.