I own a few different X68k models. It's an interesting computer, with a lot of good arcade ports and a lot of shmups. There's not a lot of variety in the library beyond action games - there's few games with depth - but the game library is fun to explore. There's a lot of weird, quirky fan-made games and the games developed by Konami, Capcom, Exact, Zoom and Wolf Team are all very good.
As far as CPS-1 ports go, they aren't arcade exact. The X68k ports throw fewer enemies at you than in the arcade games, and there's graphical breakup when there's a lot of sprites on-screen (like on the first boss of Strider for example). However, if you expand the computer out with an external MIDI synthesizer like a Roland SC-88 then you get fantastic MIDI music on the CPS-1 games that sound much improved over the arcade chip tunes.
The only other arcade port I can think of that struggles is Sorcer Striker - again, there's a lot of graphical breakup in that game, and it also needs a strong CPU to run at full speed. All of the other arcade ports I've played are pretty much arcade-perfect.
The Neo Geo AES and X68000 were targeted at completely different markets. AES was for well-off gamers that wanted an arcade in their living room, while the X68000 workstation was for people doing high-end computing work. When they were new, you could buy several AES consoles for the price of a single X68000 workstation - the X68k was thousands of dollars. In fact, X68k workstations were used by the graphic artists and software engineers at both SNK and Capcom to produce their arcade and console games.
I recommend trying out the platform through emulation first, but if you decide to go down the X68000 hardware rabbit hole, here's a few purchasing tips.
1) You can install an SD to SCSI card converter and run the entire game library off of one SD card. Highly recommended for speed, reliability and financial reasons - no messing around with expensive, fragile 5-1/4" floppy discs!
2) Every X68000 model will run 95% of the games. There are 5% that need an unusual amount of CPU power or memory capacity. The high-end XVI, Compact and 68030 models have that extra horsepower standard, but those models are also the most highly-desirable now, and thus the most expensive to acquire. You can upgrade other X68000 models to have more CPU and memory, but some of the upgrades are difficult to track down. The original X68000 and Pro models are the most reliable while the Ace and Compact models are the least reliable.
3) Like other electronics from that era, every X68000 uses a bunch of capacitors that have dried out by now and should be serviced. Later X68k models like the XVI, Compact and X68030 used surface-mount caps that leak and damage the PCB. Any X68000 that hasn't been recapped yet should have it done ASAP - especially with the power supply. The power supply is the most common component to fail, but there are adapter kits available to let you replace it with a PicoPSU.
4) Don't buy an X68000 that doesn't boot up to at least the "Insert Floppy" prompt. An X68000 that doesn't boot is very difficult to troubleshoot and revive.
5) The X68000 uses three display resolutions 15 kHz, 24 kHz, and 32 kHz. If you don't want to use a Sharp X68000 CRT monitor then you can use an OSSC upscaler with a port adapter and a VGA cable to connect it to a PC monitor or HDTV. DO NOT USE SCART CABLES! The X68000 doesn't output standard SCART voltages and it's possible to fry both your display device and a chip on your X68000 if you use a SCART cable. There are X68000 SCART cables on Ebay and they are garbage.
6) The X68000 keyboard has special keys that you won't find on any other keyboard, and some games use them. As such, while you can obtain a PC keyboard adapter and use that for getting started, you'll probably want to buy an original keyboard eventually. Plus, some games like Cotton will do cool things with the keyboard lights like pulse them in time to the music. Only a handful of games use the mouse, so you can safely put off buying one of those. The controller ports look like a Sega Genesis, but are wired to the MSX standard (2 buttons), so you'll want to either use MSX controllers, or a controller adapter that adapts a console controller. Plugging a Genesis controller in without an adapter might fry the controller.
7) The OS is a clone of MS-DOS 3 and you'll want to be familiar with DOS and a few DOS commands to use an X68k effectively. There is a nice visual menu that you can use for launching games, but if you have to do anything to reconfigure the computer or adjust configuration files, you'll want to know some DOS essentials.
Let me know if you have more questions.