My take is this:
You're in a keeper league. It seems like you need rebounds. Dwight gives you that. However, he's been hurt this year, and over the last year he's gone from a clear top 3 in the NBA talent to a mere shadow of himself. He doesn't score, rebound, defend, dominate like he used to. Probably related to injury. Big men don't have the shelf lives that guards do. Especially guys like Howard who don't rely on pure post skill, but rather athleticism. Despite how much he wants to be, he'll never be a classic back to the basket type center. It's possible his best days are past him and this year isn't just an aberration. Watch Laker games. He's not intimidating like he used to. Jefferson is on the trading block in Utah. Maybe he'll go, maybe it will be Millsap, but regardless you don't know. He could end up in a better situation, or a worse one. It's a gamble. He's not a good defender and that will certainly affect his minutes. I would say Ibaka has the most future upside. Look at him this year. He's come a long way. 4 more points and half a rebound more per game. He's still young and still getting better. The blocks will always be there. He makes his free throws too.
With all this in mind...
This year, if you need rebounding, probably Dwight. But in the future? It might be best to cut Dwight and move on to Ibaka who, at least to me, looks like a future All-Star. I've got him on my squad. Kevin Martin's contract is up at the end of this year. The Thunder haven't officially offered him a deal for the future. If he doesn't come back, that's 15 points per game that the Thunder need to make up. Remember, Ibaka is the guy they chose to pay over James Harden. OKC's got big future plans for him.