Well, the decision between RAM or Flash based was a though one, each one has its advantages and drawbacks but we finally went the Flash way because of its instant-on feature, as populating all the RAM chips on startup would take a bit, given the size of the neogeo rom data.
Say you want to load kof2003, that is around 90MBytes. The STM SD interface runs at a maximum of 25MHz, when running in 4-bit mode, you can transfer a byte in 2 cycles, so that's 12MB/s so you have an absolute minimum time of 7 seconds waiting every time the board boots, boot it 10 times, and you have spent over a minute waiting the game to load. That's a theoretical value, not taking into account the sending of commands to the SD (it uses a 1 wire protocol) and the filesystem overhead (you can only read a cluster, then check the next cluster in the FAT, and send the command to read another cluster), and the overhead of reading the data from memory and transferring to the actual RAM chips (setting the address, data, moving WE, ...) so I'd say it could easily be 15s to transfer an entire game every boot. Another issue is that the neogeo bios needs to see a valid neogeo header in the program area, so you must either first write a small loader that shows a loading screen, or keep the neogeo in reset while it's being transferred (at least the 1st MB of the P rom but then you'll have garbled graphics while they update). So we went to the flash solution, that has a slower write time, but it behaves exactly like a cart, with instant on time, having all the data ready since bootup (seeing an stock bios AES boot the game instantaneosuly is gorgeous). There are other issues but those are just some of them. From what I can see in the images of the "other" cartridge, he's also using flash chips for some parts (P, S and M roms probably, and RAM for C and V)
About the access protocol to the RAM, you can choose either DRAM (with its complicated protocol, that would need to be handled by the FPGA) or SRAM (actually PSRAM: DRAM chips with a SRAM like interface). PSRAM would be the way we would have chosen in that case, as it's more easily addressed from the FPGA and MCU, although they are usually more expensive.
Also, there are Flash chips that use a technology that gives almost instant erase time, but they aren't manufactured in all the sizes we needed.