I've been living with the RG35XX and RGB30 for a couple of months now and I have a few vague thoughts.
Overall
The first thing that's surprised me is that I've used both of these devices in different ways and neither is languishing in its box. The same can't be said for the old RG351M, but that's another story.
Having got both machines fairly recently it's taken a while to get out of the initial tinkering phase when the fun comes from setting things up and tweaking settings and whatnot.
My notes on each device are much more based on actual gaming use than hardware novelty value.
RG35XX
I played this pretty heavily early on because I'd started Link's Awakening on it while waiting for the RGB30 to arrive, and I wasn't going to change horses midstream.
The screen ratio isn't as ideally suited to the GBC as the RGB30's 1:1 aspect, but it's still a really nice experience. The d-pad suits the 2D Zelda's (basically) cardinal point movement system and there's just something cute about playing the game on hardware with the original form-factor.
I took the RG35XX on our October week holiday and found it to be an ideal pick-up-and-play device. The Min UI custom firmware is so lean and quick to boot that it can afford to shut down completely every time you suspend it, so the battery life feels exceptional. I played a lot of Tetris DX that week and although my particular device still occasionally sticks on boot-up (the reason I ended up getting it for free) it was a small price to pay for the overall convenience.
My son's asked for a retro handheld for xmas, and I've ordered a Miyoo Mini Plus on the strength of my experience with its Anbernic counterpart. The MM+ has wifi built in, which addresses one of my sticking points with the 35XX - it's kind of a hassle to curate games lists on if you need to transfer the SD every time. I think in the longer term I'll do a clear-out of my games and just leave the RG35XX loaded with a curated list of stone-cold classics I know are perfectly suited for it.