My shop has rafters on 7'-6" spacing, I went with the high rib steel, it's also 27 ga versus 29ga usually used on roofs and siding (or maybe it was 26 vs. 29, can't remember for sure - thicker anyway) .  It has been fine and I have 8"+ of insulation blown in on top of it.  You might want to look into your truss ratings to see if they were ever meant to have a ceiling load on them.  Most pole building trusses weren't designed for this.  A lot of guys don't care, but I think it might be a problem if you had a collapse and tried to collect insurance.  My rafters weren't designed for a ceiling, but the local lumber yard got some drawings from one of their truss suppliers about how to beef up the bracing in my trusses to allow for a ceiling, so I did that and it wasn't too much work for the added peace of mind.

Good Luck,

Mike