I've stayed in both many times. Although they are both attached to Pacific Place, the Conrad has more rooms with great views due to the shape of the hotel and surrounding buildings. Also, its rooms are bigger. Still, I'm often able to get better rates at the JW. While it's not as good as the Conrad, it is a solid hotel with excellent executive lounge offerings.

Bottom line: I prefer the Conrad, but if I can get a room $100/night cheaper (perhaps thru the use of bonus bucks), I'll stay at the Marriott.