I can see that both hAP ac³ and hAP ac³ LTE6 kit have exactly the same WiFi capabilities (gain, chains, max data rate, chip) but of course, hAP ac³ has external antennas.
Is it still considered that external antennas are better? Can I expect that hAP ac³ would have better WiFi coverage or at least perform better in some not best-scenario situations?
I have tested AC3 and AC2, that is why I was telling you to skip ac3. External antennae are just for show on ac3.
See http://forum.mikrotik.com/t/hap-ac/143358/23
On the other hand, both ac2 and ac3 are pretty cheap to buy, so go buy one and test if it helps you stop wondering.