We have been in both Cape Disappointment and Ft. Stevens. Both are good parks with about the same space between campsites.
Ft. Stevens is a larger park, it is spread out and it is just bigger than CD. The facilities, grass and trees are all very nice at Ft. Stevens. We did not feel crowded.
Cape Disappointment is a smaller park, the roads are narrower. It is on the water which is nice, but more so it is just different from Ft. Stevens.
Its easier to get into Ft. Stevens make reservations for Cape Disappointment.
I personally think both are very good state parks.
Good Luck!
Rick