Backpacked two summers ago in Rainier and Olympic.
They're so different that it's difficult to compare. I loved Olympic NP because you start in the rainforest - it's absolutely beautiful, especially if you're not from that kind of climate. It's crazy how green everything is. But you climb up into sub-alpine with high mountain lakes. Lots of great wildlife (saw several bears).
Rainier is one of the most impressive places I've been in terms of the grandeur of the mountain (coming from somebody who knows a lot about tall mountains, being from Colorado, where there is a plethora of 14ers (mountains that are over 14k' (for those of you from Utah))). The terrain around Rainier is incredibly rugged if you're planning on hiking or backpacking. But some really amazing country.