I think both options would be pretty cool. I have a general bias toward splitscreen (although I usually get less mileage out of those as my gamer friends live in different states).
I think there's a general lack of good splitscreen games across the board, and this could help fill that hole. I also think being able to physically see my buddy right next to me converting people could get the adrenaline going and make conversions feel more urgent. But on the other hand, online lobbies would make it more accessible for long-distance friends. I do think online mode would need more put into it to make it actually feel different than solo mode with a friend talking to you though, where splitscreen would instantly present differently as a game mode.