I've been to both and in my opinion Primeclass is much better.

I went to the Millenium lounge in December 2012. I was there from around 6 to 12 in the evening. The food selection was dismal. The only hot food were the 2 selection of soups. The other food were the pastries and sandwiches. There are also no bathrooms and showers inside this lounge. This lounge is more dim with less people and more available sofas. The service was a bit more prompt in taking away your finished plates and drinks.

I went to the Primeclass lounge this month. It was also much more brightly lit. I was there from 6 to 12 in the morning. The food was much better. For hot food there was 1 choice of soup and 3 other selections for hot food. There was also pastries but no sandwiches. They also switched the food to something different around lunchtime. There was a room for shower with amenities and towel provided. There was also bathrooms within the lounge. There are also free lockers provided for you to store your stuff. While I was there, it got quite populated at times and the noise level increased with it.

If you intend to mainly just sleep and relax, I'd say Millenium, but if you want some better food selection and get freshened up like me I'd certainly recommend Primeclass. The cost is the same for both lounges, 35 Euros.