Both will be tough opponents.
I think we match up better with Miami. We've been defending the three ball very well since the start of the playoffs. And while Butler is a very good two-way player who is capable of taking over games, he's not a physical monster like Giannis. And on the other end, I think we can have success attacking their perimeter defenders.
I don't like the Bucks matchup. We don't have a guy who can defend Giannis. Middleton turns into a freaking 6'8" Steph Curry when he plays the Celtics. They have a ton of length on defense and, unlike most teams, they have the right collection of defensive players to give our scorers trouble... though it is worth noting that our guys have been hitting from three and the Bucks have been giving a lot of those up. So there is that.
There is also the Hayward situation to consider. We have to expect that the best case scenario is that we get a 75% Hayward towards the end of the series. And it's probably more likely that he doesn't play at all. Against Miami, I think we can survive even with that limitation. But I really think we'd need him against the Bucks.