I have not been into both hotel post refurbishment. But I have been to Marriott and some rooms are quite small as the building has octagonal shape. Yes, Marriott has just completed the refubishment and even the lounge looks fresh now.
I stayed in Pan Pacific long time ago and the room is bigger. But again this is before the refurbishment but I do not think room size will change.
If you like more happening place, Marriott is the better choice as it is in the center of Orchard Road. While Pan Pacific is not really out of way also, it is more on the quitter side of the city and it is well connected to Marina Square shopping center and if you lucky you may get a good view from the building.
Like SKYravefever said, it boilds down on your preference. I think both are good hotel. Since you will be in Singapore for a week, why don't you split the stays? Perhaps later you can give us your view on which one is better?