I've demoed the akg's with some killer headphone.com amp and it sounded fantastic, very smooth and involving. I think the akg's are more prone to be picky about what is used to drive them compared the sennheisers. I've only tried the 650's briefly and thought they sounded good too. I own the 580's and think they are great, very speaker-like presentation.
My rec would be the sennheiser's because their sound scales in proportion to the amp and source used with them.