Loads better sound quality, better physical design than the current Chinese Mackies, great preamps, a very musical EQ, aux 1/2 can be combined to a stereo aux, etc. In short, for the price difference you get more than just a step up the mixer chain, you leapfrog the Soundcrafts and Mackies entirely on both sound quality and useful features, while staying in the same general price bracket.
In your case, with 3 stereos, the Zed-12 is almost perfect. It has 3 specific stereo channels, and you get the FX for free (use them if you like 'em - they’re actually pretty decent for onboard effects! or just ignore them entirely). The 14 is also great, with the extra stereo channel in place of the FX, so compare with whichever is cheaper. As a bonus, there’s a 2-channel I/O USB audio interface built in to both of them, too.
You lose nothing, you gain a LOT, totally worth the price.