So you've decided you want a Game Dad, or you're kinda leaning that way, and now you don't know which one to get, because there are scores of the damn things and each has its fanboys and you don't have enough information to make the decision.
Don't worry. That's the headline, here, at least for those of us over forty.
Do you want to play Final Fantasy 6, or Chrono Trigger from the SNES? Every Game Dad will do that. Don't stress, you can't go wrong.
Want to check out the awesome new indie Game Boy ROMs that are all over itch.io right now? The Game Boy Colour had an 8mhz processor and 32kb of RAM, a millionth of the resources of the cheapest phone you can buy; even the most modest Game Dad will open you up to this flourishing scene.
If you were born in the 80's and grew up in the pre-PlayStation days, you can choose just about any Game Dad, curl up cosy, and have yourself some 8-bit fun. It doesn't matter what Game Dad you buy. They're all great.
People online, gamers especially, get all tied into knots about whether their Game Dad is the best one - resist these feelings. The best Game Dad for you is the one that plays the games you want to play. If your main focus is playing 8- and 16-bit games, you can spend $60 on an RG35XX or $400 on a Steam Deck and you'll have just as much fun either way (although a Steam Deck won't fit in your pocket).
Don't sweat the decision. This is a funbox and the journey of getting one can be fun too. If you enjoy the process of reading reviews and making notes and comparing deals, then you can gnaw on this decision for weeks on end - but if that's a source of anxiety for you, remember, they all play games. It doesn't really matter that much.
If you want my personal recommendation, I will say that having two SD card slots has been very helpful as it lets me fiddle around with different operating systems on one card while having my ROM collection stored on the other, and the ability to run PortMaster opens me up to playing lots of cool PC games. On the other hand, if arcade games with vertical monitors are your thing, it's possible to set up a horizontal system so that you hold it sideways and can still use a joystick and buttons.
My RG353V runs both Linux (ArkOS) and Android, and in the whole time I've owned it, I've booted to the Android partition three times, one of which was accidental. There's a version called the RG353VS that omits the Android partition and the touch screen (which, again, I've barely used), and it's twenty bucks cheaper. If I were buying again I probably would have gone with the RG353VS, but I don't regret it hugely; it's only a pizza's worth of money and though it might sting in the moment, these machines last for years and I'll eat a fair amount of pizza in that time.
If you do buy a Game Dad with two MicroSD card slots (and I do recommend two slots; it makes organizing your games and tinkering with the operating system much easier because you have one card for the operating system and another card for the games) then you'll want to buy two MicroSD cards - a small one for the operating system, and a big one for your games.
Often Game Dad resellers will include no-name MicroSD cards in the package with the Game Dad. These cards are junk and will fail very quickly if they ever work at all, so you'll want to grab whatever data you can off them and then chuck them in the bin before you're tempted to put anything important on them.
MicroSD cards are so cheap these days that there's little difference in price between 16gb and 32gb cards. Everybody recommend Sandisk or Samsung cards but in my experience they're damn near impossible to buy online because there are more counterfeit or shadow-shift cards than genuine ones (Amazon is especially bad for this). Every Sandisk or Samsung card I've ever owned has failed within a couple of years, and I don't know whether this is because I'm not getting the real thing or because Sandisk stuff is overrated (and Samsung stuff is famously shit anyway).
I buy Micro Center cards in packs of five at a time because I've got all sorts of little doodads that use MicroSD cards, and I've never had one fail; maybe this is because counterfeiting Micro Center cards would be like counterfeiting pennies.
Your operating system will likely fit on a 16 gig card; check the OS maker's page to see how big a card you'll need. I got a five-pack of 32 gig Micro Center cards last time because it was 50 cents more than a five-pack of 16 gig ones.
As for your games card, well, you can buy a tiny one if you're sticking to cartridge games, but if you want to play CD-based games or PC ports, they're what'll take up the most space. At their largest, PlayStation games can hit 700mb per disc. You can fit every Game Gear, Game Boy and NES game ever released inside one PlayStation game. If you want to stick to cartridge-based systems, a 32gb card will be plenty; if you want to play PlayStation games, get something much bigger. Remember that you can always move games off the SD card onto your computer if you want to make room for something, while leaving your save files on the card if you want to move the game back later on.
I'd give you some idea of prices here but it'd be outdated within five minutes and you probably have some MicroSD cards in a drawer somewhere anyway.