O'Bryant has size and length, but he hasn't shown the ability to play the game at the NBA speed nor has he shown the physicality to mix it up with NBA bigs. You could make a case that POB is a young 20 by NBA standards and Darko is an old 21, but Milicic is the better and more polished prospect, imo.
They're both young and Darko could go either way at this point, but I think he's got the tools and the talent to be pretty good. Kirilenko is another front court player who could be undervalued this off-season. The W's would have to do their due diligence as far as the physical and assessing Kirilenko's overall health, but I think he's a guy who'd thrive in an open attack.