Salon wrote a piece on why he hasn't endorse Obama, which basically said that he met with both of them, and Hillary surprised him with committed talk about the issues he cares about, while Obama was more arrogant and would only speak in platitudes, which turned him off.
Just a couple days ago, he did a comedy bit about how he hasn't decided yet, including jokes about whichever of them get him a pair of jet skis will get his endorsement.
I don't know what his reasons are for not yet endorsing; it's possible he's not thrilled with either, it's possible he's trying to get some committments to his issues, who knows.
I do know I had preferred him to either of these two - not because of his stage abilities, his speaking abilities, but because of his clear values for the issues I like to see prioritized.
At the moment, I'm leaning towards Obama and hoping ((c) Obama 2008) his vagueness turns into something good in office, and have some reason to expect it to.
Hillary would be a lot better than a Republican and I'd certainly vote for her if she is nominated, but her history with corporate representation, NAFTA, Wal-Mart etc. is an issue.