I've been on both and it would just be my preference, but I'd choose the Noordam. I went on the Millenium BEFORE her very recent rennovation, though, and it was a MESS! I do not like that Celebrity lets the ships get that beat up before a dry dock. However, since it's been spruced up recently, I'm sure it's lovely, though I have not seen it. I was on the Noordam less than a year ago for a 10-nighter and it was in immaculate shape and the service was excellent. I don't think you can go wrong either way, but since you've asked for opinions, mine is Noordam. Enjoy whatever you choose!
We love both Celebrity and Holland America but have pretty much decided to stick with HAL for any future cruises and in fact are going on the Westerdam later this month and we were on the Noordam last year. HAL puts out such a good product for food, cabins, entertainment, ship size and staffing they are very hard to beat.