I spent about three weeks in Sabah, Sarawak and Brunei in December last year.
KK has lots to do and Sabah seems to have more things to do than Sarawak. And see the orangutans in Sabah. We went white water rafting and diving from KK and then did a lodge stay on the river. We didn't go to Danum (wish we had).
We loved Kuching. We rented a car and drove out to Baku and then to the Ai Batang (Hilton) resort which we enjoyed tho if you look on the Hilton forum, you'll find lots of people who didn't like the resort.
I'd to 2/3rds Sabah and 1/3 Sarawak.