My preference would be to pick up either Dieng or Withey, mainly because I think either of those guys already have the defensive IQ & skills that they would actually be able to get floor minutes in the NBA.
Other guys may have more athleticism, size or upside, but most of the other bigs in this range are all very, very raw - especially on defense.
And I agree with the premise that we don't want to have to carry two projects on the roster.
We aren't in need of a C who can light up the scoreboard. We have other options on the team for points. What killed us last year was lack of length on defense whenever KG sat down.
Thinking beyond the immediate need, I also want someone who looks 'complementary' to the way Sully plays. Sully already is a very strong low-post player on both offense and defense and is likely to get better and better. So you want a long, rangy high-paint defender to complement him. Both Dieng & Withey fit that profile. Dieng is better at hedging the P&R. Withey is better at shot contention. Both do a good job of disrupting passing lanes.
I'm not particularly worried about the 'too skinny' label some have pasted on these guys. They are both still 2-3 years away from max muscle mass. And bulk is less important in the high-paint role. If Fab pans out, he and Sully could provide plenty of bulk down low.
Offensively Dieng looks a little preferable to me over Withey because he looks like he has excellent form on his jump shot so could grow into a 'stretch big' role. That said, Withey didn't take enough jumpers to really say it looks good or bad. Neither has shown a lot in the way of a post up game, but both are very efficient finishers going to the basket.
Picking at #16, I think we can get a solid player. This is a flat draft outside of the top 5 or so (some would say its flat outside the top 2!). But I wouldn't have superstar expectations.
Depending on how the draft unfolds, I would not be surprised to see Danny trade down a bit to try to still get his man and add another pick, either in the 2nd or in next year's draft.