Standalone. I don't think it's worth it to pay for really bad N64 emulation (I've seen how it runs and it gave me flashbacks of how Project64 ran games back in the mid 2000s, when it was still "new"), and I already have other means to play Mega Drive games (and I honestly don't even want to play many MD games anymore, as I already played everything I wanted from it).
I just hope Nintendo can fix the emulation on the N64 because if charging ridiculous prices for a very limited selection of games wasn't enough, delivering such amateurish-level product is totally unacceptable and you guys should be better compensated for the money you're spending.