I own the 3 SNK console for about 15 years now;
- NeoGeo Pocket Color is the cheapest, and have a lot of great games (SNK vs Capcom MOTM, Card Fighter Clash, Metal Slug 1-2, Last blade, Samurai Shodown, Sonic, Faselei...). It`s would be my first pick if I had a low budget.
- NeoGeo CD; cheaper that the NeoGeo AES (cartridge version), but have least games and a loading time issue. Still, I love it to play the earliest SNK games from 1990-1995 and non fighting games (Magical Drop, Ninja Commando, Robo Army, Flying Power Disc, Sonic Wing...). Those games have really short loading time of 1-2 seconds, and you can find them for 20-50$ instead of the average 100-500$ for the cartridge version. I`ve also noticed the soundtrack of all the games are far superior (CD quality) that the cartridge version.
- NeoGeo AES; the more expensive one, but it give the best arcade experience, with a lot of really (and expensive) fighting games (King of Fighters, Last Blade, Samurai Shodown, Fatal Fury...) and shooter (Metal Slug!!). SNK supported that console from 1990 to 2004, and non official developer are still making games for that console (NG:DEV.Team).
If you want to play the NeoGeo at a cheap price, look at all the compilations and great port that came out for Dreamcast/PS2/Wii/PSP/ Virtual Console.
Few examples; SNK Arcade Classic Vol.1, Metal Slug Anthology, Fatal Fury: Battle Archives Volume 1 and 2, Samurai Shodown Anthology, many KOF port...