I didn't like the fact that for the "pc-setting" in the 8bitdo you have to use the dongle, and can't use bluetooth.
also the kingkong2 pro has exchangeable abxy buttons, so you can change them to the correct layout for the thing you're currently playing on (which may be extremely useful with the little brother, depending on his age)
From my experience, the 8bitdo Ultimate has been great. (KingKong 2 was not available when I purchased it)
I have it connected with the dongle to my PC (or Steam Deck depending) and when I want to play on PC I switch the button on the back to use the dongle. And when I want to play on the Switch, I change the toggle on the back again, and it stays paired to the Switch.
I don’t know enough about how KK2 pairs. Maybe it can do this too, not sure. I like that the charging stand allows you to keep the dongle in there and it connects to the pc through the charger.
And also, previous 8bitdo controllers have lasted me a long time.
Let us know what makes you pick one over the other. And happy gaming.
Sorry for not sticking to your to options. But. If your brother have a Switch, why not considering a Switch Pro Controller? Works great on PC, on Steam Deck, and of course Switch. I'm loving mine for PC gaming.