The kick panels are a much better choice than the doors for alot of reasons. Better mid-bass response, better imaging, better highs, better mounting point, less reverb and less sound deadening required.
I have a set of 6.5" components in the kicks and two sets of 6.5" components in the doors at the bottom. The front speakers are far more noticeable than the two identical component sets in the doors. The doors will also require HUGE amounts of sound deadening to provide quality sound and reduce the massive vibration that the speakers will cause.
My truck's doors are COMPLETELY covered, inside and out with Dynamat Extreme and will be receiving more sound deadening early next year, along with expanding foam and silicon where necessary.
