Gotta go Celtics here, by a hair.
1st: How can you call it a rebuild for Philly? They're not rebuilding much and have not been relevant since Iverson was in his prime. The Celtics were perennial contenders from 2008-2012 (5 seasons), and started rebuilding last season. Here alone you take the Celtics situation.
Celts will have a much better chance this season at winning games and a better chance at landing a key free agent next summer, presuming they retain Rondo and Green after healthy and productive seasons where the team is semi competitive.
Celtics:
1) Develop promising talent (KO, Sully, Zeller, Smart, Young, Turner)
2) Retain solid vets (Rondo, Green, Bradley)
3) Let expirings expire (Bass, Thornton, Anthony, waive Bogans -- Wallace in 2016)
4) Use 2015 summer cap space to sign key free agent / make trade if necessary (ideally Gasol, Asik .. .player of impact and need)
5) Continue injecting talent via plentiful draft picks over next 4 years while winning or at least being competitive
Philly:
1) Develop promising talent (MCW, Noel)
2) Pray / wait on promising talent, then develop it (Embiid, Saric)
3) Lose like crazy again this season and pray for lotto balls (get top 3 pick) and hope that pick winds up panning out over the next 5 years
4) Try to unload Thaddeus for picks and let JRich expire
5) Continue losing again the season after that for more lotto action unless some free agents of note decide they want to gamble on a great opportunity/take money in Philly
I hand it to Philly for swinging for the fences and not even trying to pretend they want to win. Their gambles could pay of big time. It'll be interesting.