The interface of the three EnGenius models don't differ much, and we're generally pleased with the user experience.
There are a few differences, but that's because one model has 5 GHz and the others don't. And the models without USB obviously don't have an option to configure the port.
The different models do have a couple interesting features. All three have a VPN server, something you don't see often, especially now on entry-level models such as the ESR-300H. You can also create 4 SSIDs per frequency, which can then be used for guest networks.
Other than that it's about the same that you find on other routers. On the two models with USB ports you can choose whether they can be accessed only via software you install on your computer, or by anyone. If you turn off the NetUSB, the connected storage media can be accessed via SAMBA. It's similar to D-Link's SharePort Plus, except there you can't turn it off.
The ESR-750H does have one unique feature called shAir, which allows you to play music from AirPlay on speakers you connect via USB on your router. It's different than the AirPort Express, that actually has an audio card with connectors. This router apparently has a generic audio driver built in to the OS.