As an owner of both the Supercard DSTwo and an R4i Gold 3DS RTS, I can say that indeed, the DSTwo has a lot more features, but for the vast majority of users, doubly so for softmodded 3DS users, an R4i Gold 3DS RTS/Plus is not only fine but actually ideal.
See, a lot of the additional features offered by the DSTwo have been outdone by alternatives available elsewhere. If you're on a modded 3DS, you can use Open AGB Firm or custom VC Injects for cycle-accurate GBA games, and the port of snes9x by bubble2k is more than adequate for SNES games. Certainly a lot better than TempGBA and CATSFC.
This leaves the only real extra features are... being able to read text file guides and view bmps while in-game, and the freecheat features. I can't argue, if you need those in that exact fashion, the DSTwo is the only way to go, but if you ask me, it's a lot easier to develop cheat codes
for NDS via an emulator like DeSmuME on a desktop PC with full functionality of the Action Replay code format, rather then just lazily making codes on the fly in-game. As for guides, like, it's 2021 dude, I'll just pop out my phone or use the nearest computer to look things up.
The claims about game compatibility are nonsense btw, I tested nearly 300 of the most popular Nintendo DS games on both, and as far as I can tell, compatibility is more or less identical between the two- actually, I found a couple of obscure kids games that oddly wouldn't work on the DSTwo that would on the R4i Gold 3DS RTS just fine. So this is a moot point, anything you want to play pretty much is going to work regardless of which you have.
On that note, the downside to the DSTwo often mentioned is the battery drain problem.
This is a real problem
and I know it is because ran some tests on it. I don't have the datasheet anymore since this was quite some years ago at this point, but from what I recall, the conclusion I found is that It is much more noticeable on any of the 3DS models, due to it drawing the extra power even when it's not in active use, and the likelihood of it being inactive is much greater on 3DS units since unlike any of the DS or DSi units, you also play 3DS games on a 3DS, often digital titles on a softmodded unit.
The issue comes from the fact that the 3DS already draws a lot of power, and the DSTwo, being that it runs a custom processor inside which is powered on no matter what while it is plugged in, it shortens battery life quite significantly. I ran some tests and I found that, in 3DS games running in 3D, I lost 30 to 45 minutes of playtime by just having the DSTwo inserted over leaving the slot empty or populated with a 3DS game. With the same test but with an R4i Gold 3DS inserted, I didn't notice a change that wasn't within the margin for error. (less than 5 minutes difference or so over a random 3ds game inserted).
That's quite a significant dip, and it's the main reason I switched to using an R4i Gold 3DS RTS in my old daily driver 3DS console.
The R4i Gold 3DS Plus also has much wider availability, since I'm not even sure if DSTwos are being manufactured anymore. The conclusion I've drawn is that it's best to use an R4i Gold 3DS RTS/Plus on any softmodded 3DS. The DSTwo is still fine for 3DSs too, but minimize any time you spend with it inserted but not being used if you don't want to suffer needless (and significant) battery drain. All of It's extra features are rendered useless by other modern coveniences and superior alternatives.