focal length ken, the 900mm will give more magnification but like adding a brlow to your scope it makes the image dimmer and narrower too. the 600mm will give a brighter image and wider, but the trade off is less magnification.
the mounts are probably interchangable even tho the f6.6 is shown on a az mount and the f10 on an eq.
so yeah the 600mm is better for wider field and deep space i'd assume