Zeller has a RB% of 14.8 this year compared to Plumlee with 17.3%. They both have a DRB% of around 20% but Plumlee has an ORB% of 13.3% to Zeller's 8.8%.
I agree Zeller is slightly more versatile offensively, but I don't think he doesn't anything so spectacular that it makes up for being considerably worse as a guy to lock down the paint and win the rebounding game.
Zeller is so much better than I expected him to be. I basically only watch Plumlee when we play them, but it's (barely) enough to form a few opinions. I like him, too. I like the physicality he plays with. He's a better defender than Zeller, but Zeller's no slouch there, either. Zeller's more refined offensively. There's not huge separation between them in my book, but if I had a gun to my head, I'd probably opt for the defense, which means Plumlee.
Now, a more interesting question is Plumlee or Low T (Kelly). We could have drafted Plumlee instead of Kelly. I understand that Kelly has skills, and if he had toughness to go along with them, I'd easily opt for Kelly. But I just don't like soft bigs, and Kelly is just a better version of Brad Lohaus in my book. I like a big to play like a big, not like a finesse softy.