The most expensive and, in theory, the most powerful out of the three EnGenius models is the ESR-750H. It looks exactly the same as the ESR-600H, and if if it didn't have a different product number on the bottom you wouldn't know the difference. Of course it's not uncommon for a manufacturer to use the same body for multiple products, but it doesn't feel that you have a more advanced router than the 600H even though it's £40 more expensive.
It turns out that not only the casing is the same, the 750H also has the same PCB as the 600H. You can tell by the serial number. The only difference can be found in the upper right-hand corner, where the 750H has an amplifier. That's in order to create the theoretical maximum speed of 450 Mbps. The 5 GHz band is provided by the Ralink RT3883F, which also serves as CPU. For the 2.4 GHz band the ESR-750H has the same chip as the ESR-600H, the RT5392L. The switch is also identical, the Atheros AR8327.
It's a bit odd that the ESR-600H and the ESR-750H is use the same chip for the 5 GHz band and the CPU. All other routers we've seen that use a shared chip like this combined the CPU and the 2.4 GHz band, not the 5 GHz band. It's difficult to compare, but it could be an indication that EnGenius views 5 GHz as an afterthought. The fact that the large external antennas are only used for 2.4 GHz do seem to support this. It's not a terrible design decision per se, because it's still much more important that you have a stable 2.4 GHz connection than one on 5 GHz. The 5 dBi antennas should also create a good range at 2.4 GHz, something that isn't possible with 5 GHz anyway. On the other hand, if 5 GHz isn't taken serious by EnGenius, then it's a bit peculiar that they still created a model with three data streams. In every other aspect the ESR-600H and the ESR-750H are the same in terms of hardware.