Personally I don't think that the E-series has better build quality, as the N91, N91 8GB, N70 , N93, and N93i proves this.
I also think that Nokia pioneers new formfactors in the Nseries, by the time they use it in the Eseries it's been tried alot.
Generally I think Nokia should go back to the N70 days and make those solid feeling phone. The N95 is no excuse, especially looking at the excelent quality of the E65. On the other hand the E65 doesn't have a dual slider.