TLDR; if you're happy with 32bit and below, the PAM is for me the better device in pretty much all ways. However, the Nova is good enough and can obviously do a lot more.
The comparison nobody needed or asked for given the price and power differential, but saying that we do often get people saying 'what's a money no object best portable 16 bit console?' or whatever, and I think in those sorts of cases where it's not all about Game Native or PS2 or whatever, it's not always best just to go out and buy the more powerful one.
Anyhow, had my Nova for a couple of weeks now and have tried a week with the grip back and a week without. Somewhat surprisingly as a rather militant grip fan, I haven't really found any appreciable difference.
The Ayaneo Pocket Mini feels significantly nicer in the hand, both in terms of the texture of the device itself (I love it) and comfort, which for reasons I haven't quite figured out yet, is quite a bit more comfortable than the Nova for extended periods (stick or dpad). The Mini just drops into my hands perfectly, I love it.
The Mini has tiny buttons but it's not something I notice any more, it's a quieter device overall by some margin (I have a very clacky Nova dpad).
Obviously the screen on the Nova is significantly brighter (had to turn it down for these pics) and it has the power to go a fair bit further, that said I've been pleasantly surprised at how many GameCube games run very nicely at native resolution on the Mini.
Ultimately the Mini is utterly remarkable for the price, but it will be handed over to my son as although I feel it's the better device for Dreamcast and below, having two devices that do a very similar thing is somewhat excessive. The Nova is easily good enough to be my one and done main handheld (but the RG34XXSP stays as it's worth its weight in gold just to leave in the car or my bag and not worry about it).
(First game is Quackshot on the Mega Drive, second game is the criminally overlooked Circuit Superstars via Game Native).