What about the actual FPGA specs? Is it more power efficient per LUT? A newer generation or the same generation but a smaller chip?

The Pocket has an FPGA from the same series as the MiSTer, but it's a lower-end model that's optimized more for power consumption. Most notably, it's missing the 800 MHz ARM core (meaning that it runs a custom "OS" off of a microcontroller rather than Linux like the MiSTer), and it has a bit less than half the amount of logic elements. This means that 16-bit console emulation is about the highest you can go on the Pocket, while the MiSTer can emulate the Playstation, Saturn, and N64.