The swappable skillets (like the globe) is where the KISS AC differs from the gen3 KISS stuff.
KISS carts are (somewhat) disposable. You can't just buy a new skillet when it breaks and put it in there, you have to buy a new cart. They are possible to rebuild, but require skill and some parts that can be tough to acquire.
AC is globe style, where the skillet can be swapped on the fly (when it's cool enough to touch!). It comes w/ 3 different skillets and both globe and cart options, so it's a bit like a modular KISS cart.
As far as performance comparison, I have only tried the 3rd gen KISS cart a few times, but it's comparable to the AC. Like
@StormyPinkness mentioned a bit back, there's no oil "creep" like the KISS mini suffers from, at least not right away. I put in a good 0.15-0.2 which it vaporized completely in a second or two long pluse of my HVD top's button. There was no splatter anywhere to be seen, which surprised me. Not sure if the airflow is any different between the carts, but MAN that seemed efficient. Also the 2-part construction is much nicer, IMO, than the old 3-part.
The 3rd gen KISS is way smaller than the AC, and lighter. So if you wanted something inconspicuous and highly portable for a small pen, I'd go 3rd gen KISS, keeping in mind that the unit will need replacing down the line. If you wanted more options, and longer product lifetime I'd go KISS AC -- All the skillets for the AC can fail, but at least you can swap new ones into the same unit. And buying them in bulk is cheap, unlike the 3rd gen KISS carts.
Also of note: my KISS AC gets wicked hot. The 3rd gen cart has been used only a few times, and not for sessions, so I can't know how hot it CAN get, but it didn't retain heat the same way.