You're a nimrod! just kidding.

You can fit "most" 6x9s in the front door without modification. The rear requires some mods on the door panel to make them fit.
If you decide you want to go with the other two choices you mentioned, I would probably recommend the polks over the Pioneers...Will they sound the same?...probably close but all speakers have their own sonic characteristics that are created by cone materials, surround materials, tweeter types, etc...