The two are pretty different systems at the moment, or at least until Ubiquiti fleshes out the AC firmware more.
Some main points:
- ePMP is a PtP and PtMP 802.11n based product that covers both FCC/US 2.4GHz, and 5GHz covering 5.1-5.2,5.4,5.7GHz.
- Airmax AC is a a PtP 802.11ac based product that currently only covers the 5.7GHz band, IIRC they're still waiting for certification for other 5GHz bands. They're PtMP firmware is still in beta IIRC.
- There are a number of confusing options for Airmax AC... e.g. there's one that just does PtP, another that does PtMP... or is expected to do PtMP when the firmware arrives, there's also ones with the 'air prsim' and others that don't have it, there's others with an off load chip and others that don't. All in all, I find it quite confusing, and UBNT's marketing materials don't help. You just have to read ALL the threads very carefully in the UBNT forums and look for UBNT staff comments to figure out what does what.
- Airmax AC is not backwards compatible with older Airmax TDMA, supposedly they'll introduce compatibility, but there's no timeline IIRC. Based on Ubiquiti's track record, this might never come to pass, or possibly will, but will be very limited/kinda broken (like they did with the old 802.11b/g radios to make them compatible with Airmax).
- There is no 2.4GHz Airmax AC product, and there probably never will be.
- ePMP supports frequency re-use and GPS synchronization, Airmax AC doesn't, and Ubiquiti has mentioned that it never will support re-use or GPS on the Airmax AC line
- In order to get any benefit from "AC' e.g. 256QAM, your SnR needs to be very good, otherwise, it performs just like any other 802.11n radio. In a high noise environment, AC is pretty useless.