I think of what the bees would prefer. I can cope with 14 x 12 frames with no more trouble than deeps, but the boxes are much heavier to lug around (who would move a deep and shallow as a single lift?), but the bees are more important than the beekeeper having an easy time!
Wintering on 14 x 12 means one box. They do not need to move between tiers of frames all year round. But there will be more honey in an extra deep at harvest.
How many eggs your queens can lay in a 25 day period is the real criterion re number of brood boxes. I find that a single brood box is adequate for all but the most prolific bees at peak laying time.
Of course, if your queen lays 5000 eggs per day (yes beano has claimed this, before now) you may need more than 70,000 brooding cells unless, as beano’s queen did, she supposedly stops laying after ten or 12 days and takes a rest!
Overwintering on one full box is, IMO, better and easier than two - and with 14 x 12 boxes, the bees can cluster that much higher in the frames in cold periods. No risk of forgetting to remove a QE, either. If frames are fed back under the brood box in the autumn, they can be spread out (deters waxmoth) but left over the winter without any fear of being brooded in before the first spring inspection, while providing extra protection from any turbulent draughts around the OMF. Somewhere to store a few frames, too.
Old deeps can simply be converted to 14 x12 by fixing an eke below the box. Easily reverted to deeps if one decides the format is not for you. A deep and a shallow will accommodate brood frames as an emergency extra hive , if necessary.
Most ways round, an improved home for the bees - and that is not taking into account all the extra disruption inflicted (on the wax between boxes) at every inspection.
Your choice. I changed to 14 x 12 after my second year. Went top bee space, too. I don’t intend changing back, even though I am another 15 years, or so, aged and not even able to carry a full shallow of honey too far these days.
