Acknowledging whatever bias I have, I think the Celtics have the better roster.
Bradley, Crowder, and Horford are all top defenders in the league and all averaged 14+ ppg last season for playoff teams. The only guy in the Celtics' starting lineup who isn't a two-way player is IT but only because of his size on defense. He's still active defensively and knows where he should be.
In OKC you're starting the #11 overall pick and at least one guy (Adams) who is a standout defender but lacks a reliable offensive game. Waiters, Roberson, and Kanter are all guys who only play one side of the ball (and I personally don't like Waiters as an offensive player, either).
The Celtics also aren't done. They've got a ton of assetts to bring in another impact guy for the bench. OKC doesn't.