Interesting clear days per year stats here: http://www.wrcc.dri.edu/htmlfiles/westcomp.clr.html

Seattle : 71
Portland : 68
Spokane : 88 - dang I thought it was sunnier than that out there.
Los Angeles : 186

Seattle vs. Portland rainfall is about a tie. 35.5" SEA, 35.7" PDX.

Seattle is much less likely to experience oppressive heat in the summer than Portland.