It would be obvious to do it during Nintendo's conference - but it would be more interesting if they confirmed it during Ubisoft's.

As Nintendo - I think you'd want this as you'd capture an audience that may not be watching the Nintendo spotlight later that week. You get some extra press for a given day of E3 without needing to do anything.

Ubisoft are the ones making the game. Let them explain it.

Even though it's a known title - it still makes an intriguing headline for Ubisoft to announce a Mario game rather than Nintendo.

Hopefully at Ubisoft's press conference so that Nintendo gets more face time outside of their own events and so that (most importantly) the game doesn't take up a significant chunk of time during Nintendo's short spotlight event.

I think it's very likely to be shown at Nintendo's presentation, but I'd like it to be shown at Ubisoft's because that way we have more time for new announcements from Nintendo =P

Hmm, you make a really good point, though I do feel it's more likely to be shown at Nintendo's presentation, although it would perhaps be a better move for it to be shown during Ubisoft's. Guess we'll have to wait and see.

Probably Nintendo cause Mario brand > Rabbits brand

I can't recall a single collaboration (Hyrule Warriors, Pokken Tournament, Tokyo Mirage Sessions, etc.) that wasn't announced through Nintendo.

Nintendo's stream.

This is a Switch exclusive game- it's going to be shown at Nintendo's spotlight. That's what happened with monopoly, lol. If it's not coming to other platforms why should Ubisoft present it? It's only going interest Nintendo fans.

I'd be very very surprised if it was not on NIntendos stream

probably at Nintendo's presentation, would be surprised if it wasn't actually