I have a Subway 2x12, and I have two Subway 1x15's. What you get out of the two 15's is more volume (obviously), more bottom end but very tight and focused, and plenty of mids and highs. My favorite sound so far (in a 50+ year search). But, what you get with the 2x12's that you do not get with the two 15's is much more, "natural" mids and high mids. Not "honkey" by any means, but plenty of mids. When I use the 2x12 alone, I usually add a little bass or turn on the DEEP switch and add more HPF to the mix, and I generally bump down the mids just a bit. Even so, when you get to those funky, slapping-n-popping songs, the cab really shines.
I use the two 15's with my classic rock band, and I use the smaller cab with a smaller, more acoustic group, although this weekend I used the little cab with the full band and it did extremely well! At first the guitarist commented on how small the cab was, and afterward he commented again on how good (and how loud) it sounded. I use the 2x12 rather than using a single 15 nowadays. I just prefer it - but I do not prefer it over two 1x15's.
You cannot use the same amp settings for both cabs - too mid-rangy, not enough bottom, or too much bottom and needs more mid-range, depending on the cab(s) used. But, both (2x12 and/or two 1x15's) cabs are excellent! I have no complaints.
Oops. Forgot. D800+ head.