Yeah. July has a marginally stronger undercard, less "mindie" bands as they are being referred to. It's really hard to make broad generalizations about either lineup though - I honestly feel as though they each contain something for the various types of Squatchers.
July did do a better job of obtaining bands that aren't playing other festivals, especially with dem headliners. I'm lucky that Kraftwerk and New Order are two bands I've always heard are fantastic but never got around to checking out. I'm also one of three people to have never listened to In the Aeroplane over the Sea to completion, which aside from essentially discrediting everything I have to say about music, also saves me some pain at missing NMH.
May's headliners are three groups I absolutely love, and that hasn't happened to me since 2010. Both the National and Queens I've seen before, but saying they're both excellent live bands doesn't do them justice. Outkast is throwing a party at the Gorge and everybody's invited.