do both of them uw as much as you can, and do nbme as well. by the way what sort of package did you purchase from uw, coz i purchased one and on some of the topics i can only chose 40 to 48 questions max.
Do all of UW and learn the explainations well, it will be the best prep for step 1. Afterwards, use the NBME questions to gage where you are at. Most people who I'm talked to in my class agreed the NBME questions were a good predictor of their final score.