It's a tough one, a lot might depend on what condition Porzingis and Lively are in because one of them being fit could be the difference maker.
Otherwise, the biggest advantage for the Mavs is that they have the two best clutch players and Boston have looked a bit shaky mentally - you don't know how they'll react when Dallas go on one of their runs. Biggest advantage for the Celts is that in Holiday, White and Tatum, they've probably got the best collection of players in the NBA to guard Luka and Kyrie. It could be a classic, that's for sure.