I don't have an N85, although I've handled it in stores. I can say though that when it comes to build quality, I don't think anyone will fault the N86. I've pressed all over the device and there are no creaks, the slider is SOLID, keypad is discrete and large enough for men's fingers, backlighting is even, the screen is brilliant... about the only niggle I have about the way the N86 was built is regarding the camera button. It isn't as easy to half-press as the N82's camera button was, although I think it's getting better as it gets less stiff the more I use it.
Oh wait! I just remembered there IS another thing I hate about the N86's build. The SIM card slot is horrible. I have to use tweezers when I remove the SIM card.