I think both are phenoms but the edge might go to Danny.
It's easier to appreciate Brad because we see what he does on the court every game. He gets much of the press and guys seem to genuinely love playing for him. I'm not taking anything away from Brad because he's the man.
But Danny may operate at a different level. Take the Stevens hiring. Danny hired him at a time when no one was hiring college coaches. Not only did he hire him, he gave Brad a SIX YEAR contract in order to lock him up long enough to turn the team around. Most GMs would have given Brad a 3 or 4 year deal and thus it would have been a constant concern about "is he returning to college?" or "will he jump to a better team?". Danny took all that out of play by giving him the 6 year deal which gave Danny enough time to field a team that Stevens enjoys coaching, allow Brad to prove himself and then inked him to an extension. That was a pretty ballsy move.
In another example, I did an analysis of the minutes for this years' team and I don't think it's accidental that the numbers work so well.
The total minutes played last year (which would factor in any injuries last year) for the current Celtics squad, giving Tatum 20 minutes per game for 70 games (1400 minutes), is 19,744 minutes played. There's 19,680 available via math (48*5*82) and the entire Celtics team played 19,755 minutes (which includes rounding) last year.
Guys like Brown and Rozier are expected to have more minutes this year. Brown is likely to come close to 2,000 minutes this year (+500 conservatively) and Rozier should top 1,500 (+250). So now we're at 20,500 minutes played when there's only 19,680 available. And this doesn't take into account guys who are expected to contribute like Semi, Zizic and Theis.
What does all this mean? Danny has assembled a team where suffer normal injuries and not be forced to start young guys who aren't ready. But if/when those guys are ready, he can plug them into this roster and reduce the workload on his stars - saving them more for the playoffs. Odds are that at least someone like Semi, Zizic, Theis or Yabu can contribute sooner rather than later.
So he wasn't afraid to turn over the roster of a 52 win, ECF team from the back end because he knew he had the front end covered. He filled the back half of the roster with talented young guys who will earn minutes the hard way by going against a talented core every single day in practice. We'll see the cream rise to the top and we'll be stronger for it in the long run.
Either that is one major happy accident or it's a brilliant piece of planning.
Then there's trades for guys like Crowder and IT. Those weren't accidents. So while I don't always agree with Danny's moves, he's right far more often than not and sometimes he seems to operate at an entirely different level.