I’ve pasted the specs below, but in my opinion the biggest difference is that the OpenFPGA - in its Analogue Pocket form - is an end-user friendly target. MiSTer is more “enthusiast-friendly” with more options and upgrades (including an recommended add-on to the basic kit).

MiSTer “tech specs”:

Intel/Altera Cyclone V SE (5CSEBA6U23I7) FPGA SoC with 110,000LE (41,500ALM) and 5,570Kbit of Block RAM.

ARM Cortex A9 dual-core CPU at 800MHz.

HDMI video and audio allowing easy connectivity to any modern monitor/TV.

1GB of DDR3 RAM that is directly available to both ARM and FPGA.

High-speed ARM <-> FPGA interconnect due to both being in the same chip.

OpenFPGA specs:

Intel/Altera Cyclone V FPGA 49K logic elements and 3.4Mbit BRAM

Intel/Altera Cyclone 10 15K logic elements

2x independently addressable 16MB cellular RAM (128Mbit x 16)

32MB low latency memory 1x synchronous DRAM 64MB (32Mbit x 16)

1x asynchronous SRAM 256KB (128Kbit x 16)

You can't carry a MiSTer in your Pocket, but you can Dock a Pocket.

It doesn't support a few power-hungry systems supported by MiSTer.

MiSTer doesn't support OG physical carts.

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.