Personally, I think we would be better (and instant favorites in the East) if we got Durant and Dwight...but regardless of whether it is Durant/Dwight or Durant/Horford, that would be a REALLY good team.
This would mean we have Durant as our #1 scoring option (which is optimal for him), Thomas as our #2 scoring option (which is optimal for him), and Dwight/Horford as our #3 scoring option (which would be optimal for either of those two guys) and Bradley as a #4 option (which is perfect for him).
Also with so much attention diverted to Durant and Thomas, Horford and Bradley would find themselves wide open almost constantly - and they are both VERY dangerous when left open. Crowder is no slouch as your 5th option either
There is really not a single guy on in that lineup who you can afford to leave open, even for a moment. That is a really, really tough lineup to defend...and yet the defense (with Bradley, Crowder and Horford) would be pretty tough too. Then on top of that you still have the likes of Jackson, Rozier, Smart, Brown, Olynyk, Mickey, Zizic/Jerebko/Amir (if retained) coming off your bench. Then on top of that you still have two more potential top 5 picks from Brooklyn in 2017 and 2018.
I'm honestly not sure if Cleveland would be capable of beating us in a 7 game series if we got Durant and Horford, and I'm not exaggerating here. That would be a really, really hard team to beat for a number of years - and we'd have enough young talent to keep us competitive even after Durant/Horford start to fade out.