Spring for trusses, they save a lot of work, time, and are stronger than site built rafters. Be sure to tell the guy you are ordering from that you want to put a ceiling under them. If there is an option like 25 Lb per sq. ft, vs 30 lb/sq ft, go for the heavy ones, the difference is only a couple bucks each, I do mean like $2 or $3.
When you put the decking on, use twist shank nails--they just plain do not back out--period. A tornado came and lifted my roof off in one piece. Hired a crane to come and put it back on after I built new walls.