Hi,
I'm UA Premier Exe and Delta Gold and fly a lot from Europe to the States. I think that UA give more special care to the Premier Executive than Delta to to Gold Medaillon.
Of course it's very nice to use the lounge on Star thanks to United.
But with Delta you receive 100% Miles bonus with all the Skyteam members (Air France, Al Italia, Korean an Cz) you don't receive that with Star Alliance.