The Taylor 612ce is a Taylor 12 fret where essentially they restructure the relationship between the neck and the body and how it is joined to allow a different pattern bracing and change the location of the bridge. This essentially heavily boosts the mids, making the guitar punchier. The body is also smaller, and the scale length is shorter, meaning this guitar is built to be comfortable and unobtrusive in your hands, but the redesigned bracing and optimized to boost the volume and warmth to mean this little guitar is mean, and punches above it's weight class!
The Sitka Spruce and Maple combo helps to balance out the tone a little bit, giving the lows and highs enough presence to accent the natural mid heavy nature of this design. Taylor also uses a heat treating process to help age the top a bit, opening up the Spruce a bit, to help this guitar come in with the legendary sound of aged Spruce.
So both of these are great guitars that are pretty heavy on the mids, smaller, but not lacking in power, with a short scale length and both are super comfortable to play on. So what is the difference. Well, if you are looking for something that is really embraces that mid heavy identity, and is really specialized in that area, the 322ce SEB is going to be a great guitar for you. It really ticks all those boxes and does a great job at all of those things. If you want something that has all of the playability, and still has a really nice mid-heavy sound but is just a touch more versatile, with a little more balance towards in the overall EQ of the tone, the 612ce may be your best bet. Either way, both of these guitars are really great and something anyone could Fall in Love with!