I have both a 12000 and a 15000. I bought the 15 mostly for the WiFi which I am so glad that I did. I use WiFi functions all the time especially AcuSetter app.
Differences: 15000 (and I assume the14000) has a much nicer screen and easier to use. It doesn’t have that wasted space for that touch dial at the bottom of the screen.
more decorative stitches and Tapered stitches
HP foot and plate for phenomenal straight stitching(!)
Automatic needle threader
The hoops for embroidery are the same.
I don’t know the price difference between a 14 and 15k but because I like and use the WiFi, for me the 15k is a better choice over the 14k, and if you already have a 12k, moving up to a 14 won’t be that huge of an improvement, except for the HP foot.
I actually prefer the manual needle threader on the 12, and the positioning of the other function buttons (thread cutter, foot up) over those on the 15k.!!
That’s my opinion anyway!