I did two Assura RCX extractions with "coupling" and "de-coupling" extraction. The post-layout simulation gives pretty different result. The "de-coupling" one works fine, but not the "coupling" one. So which extraction is more accurate? Thanks.

you should use coupled one.. it is more accurate as it derives the parasitics between two nets.....

in decoupling mode.... it get thee parasitics between nets and decoupled that to ground....