This has been a very good thread.
'Might as well throw my $.02 into it. I'm going to compare these guys to one of our current 6' 9" players.
Vonleh has been one of my draft binkies for a while. His physical upside is very, very alluring. He has a nice shooting form. Offensively, his game is actually similar to Jeff Green. Yes, he's got a bigger body and a couple more inches of wingspan, but like Green he is an outside-in scorer. He has a nice outside shooting touch and when he drives, if he can elevate anywhere within a few feet of the rim, he has the touch to put it in the hoop. He's not the transition weapon that Green is, though. But he has the potential to learn to use his bulk in the post. If he does, he has the potential to be an elite scoring PF.
Defensively, though, Vonleh is nothing like Green. He is mostly a lost babe in the woods on defense. Among the top 10 players, I think he may be the _worst_ at getting caught flat footed and out of position on P&R defense. I can only hope this is due to how young he is.
Gordon is like a mirror image. He's got comparatively average physical tools, but does make good use of them. Gets up & down the floor quickly and uses great fundamentals on defense to make his reach seem longer than it really is. Great at shot contention. Defensively, Gordon reminds me, again, of Green. He's extremely good on the perimeter, plays great team defense and generally is always in the right place to make a smart defensive play. Like Green, he is prone to getting pushed around by big bodies, though. And because he tends to be keep his man outside, like Green, he is rarely in position for defensive rebounds. He's good at causing misses, but he needs his teammates to grab those and help block out weakside crashers.
Offensively, though, Gordon is nothing like Green. Gordon's only real scoring skills in the half court are inside-out. If he gets a mismatch, he can score near the rim and he's a fairly smart passer. So he hangs around the baseline and cuts inside a lot. This does get him in position for a lot of ORBs. But ultimately, Gordon has terrible shooting form, from pretty much everywhere on the court, including the FT line. He looks just awful with his wrist action coming so late in his motion that his shot is wicked flat. He has no touch on the ball when taking shots in traffic. I can only hope this is because he, too, is so very young.
Basically, both these guys have serious talent and yet also serious holes in their games.
The thing is, as has been mentioned by others, they are both very young. Two of the youngest players that will be in this draft (assuming they both declare). There is a reasonable chance that each will eventually be able to fix the weakness in his respective game. But I don't think either will be a complete player for at least a couple of years, maybe longer.
If they both reach their upside, where Vonleh learns how to play defense and Gordon learns how to shoot, then both should be solid starters on good teams.
But if they don't fix the holes in their games, then Vonleh turns into that classic high-scoring/no defense 'star player' on a perpetually bad team and Gordon becomes that defensive specialist off the bench for a good team.
Bringing the discussion to the Celtics (and ignoring that trades could make all this moot): Neither is likely to be as good as Sullinger as a starting PF. Gordon could immediately be a useful backup behind Green at SF - a guy who won't hurt you when assigned to defend the position that is usually the best scorer on the other team. He's still slight of build, but he wouldn't need to put on much weight to play SF. So he may be the more immediately useful of the two. If he learns how to shoot, then he becomes a home run. If not, you'll at least get your value out of him on defense.
Vonleh has the higher upside as a player because of his scoring potential, but if he can't play defense, he could take years to get to the point where you'd trust him on the floor if you are a 'good team'. But at the PF, I don't see how he gets time on this team, barring major roster changes. And he may never learn to be a good defender. So I think he also has a lower floor than Gordon. Although, you could probably play him enough to get gaudy stats and trade him.
My gut expectation is that if Danny doesn't get a top-5 pick, that he probably trades out. I don't think he's looking to draft a guy who will take most of his rookie contract to develop. But predicting Danny is a fool's game.