I've been seeing this question a lot lately. My answer? None of them.
Well, I take that back. If you're looking to buy something because it might have value later, then you shouldn't bother. If you see a collectible that you really like, by all means, go for it. If you can afford it, that is.
It seems like everyone and their brother is currently selling some sort of Obama/Inauguration collectible. I've seen everything from t-shirts to thongs to buttons to pictures, and you can buy them everywhere - infomercials, vendors, specialty websites, and even at the grocery store! Sorry CVS, but I don't need your commemorative Obama plate. The Obama coins being marketed? Those are not made by the U.S. Mint. Well, they are, but they're just U.S. coins that have been altered to put Obama's picture on them. They're not official commemoratives. So they don't have any added value either. Though at least they're worth the coin that has been altered, provided you can get someone to accept it.
I caught a clip on the news a few days ago with a collectibles expert discussing the current memorabilia available and his advice was what I stated above. Don't buy something because you think it will be valuable later. There's so much out there that it probably won't have much value at all, at least not during your lifetime. But if you want something that you can treasure, something you can show your grandchildren, then yes, go buy that Obama button. Wear it proudly. Just don't expect to sell it for $500 down the road.