Ground-mount arrays can always be aligned so as to face south at the proper angle. This can be important if the roof of your house is not well angled for the sun. Ground-mount gives you easy accessibility for maintenance, and the architecture of your house is not marred. It can also be placed far away from the house. But it also might require a protective fence.
Roof-mount arrays are cheaper than ground mount, more easily avoid tree shade, and might be the only choice when you have limited yard space. The weight is not a significant factor; each 30" x 60" module weighs 30 pounds and generates 200 watts, so a typical installation of 25 modules generating 5,000 watts would weigh 700 pounds, plus the weight of the roof racking.
There are two kinds of sun-tracking arrays: motor-driven arrays, which use up some of the solar-generated electricity, and passive Freon-type systems that use differential heating of gasses to tilt the panels over the course of the day. Sun-tracking arrays have moving parts and therefore wear out faster than fixed arrays. But tracking arrays generate up to 25% more energy per module and are useful in situations where the most power has to be generated using the smallest panel footprint.
