I have a few questions and observations that will effect what you should do. First, I see that the peak (ridge) seems to be over the existing framed and drywalled wall. What is on the other side of this wall? Is this garage attached to the house or free standing? Do you have or intend to put any heat or heat dump register in this space?
I also noticed that the roof framing is standard trusses, not attic trusses. Where is the attic space going to be located? If the attic space is going to be over the bay you pictured, then you are limited in the amount of insulation you can install and still have a floor for storage. If the collar ties are 2X6, then the most blanket type insulation you can install is R19. 2X4= R11. Fiberglass blanket would definitely be the most cost effective in a small space such as you have. I would use unfaced F/G, and a 4 or 6 mil plastic vapor barrier installed before you drywall. As for installing drywall, you really should install strapping at a max of 16" on center to mount your 5/8" fire rated rock to. 24" inch on center is too large for hanging rock overhead, especially 5/8" rock. If garage is attached, 5/8" drywall is minimum code in most areas.
I also noticed that you have a metal roof. Metal roofs are great, but are notorious for condensation. This makes it absolutely necessary to have a good vapor barrier and good venting to avoid potential interior "rain storms" lol.
My other consideration would be insulating the exterior walls. Since there seems to be only one long wall and very little framed cavity wall on the gable ends, it would be advantageous to go ahead and finish the job by insulating and drywalling these walls. If you only do the ceiling, you will not have a good barrier to air, heat and humidity leaking by the ends of the ceiling at the wall plates and entering the attic area. This kinda defeats the whole idea of what you are doing.
Moist air is condensing (barring any oughtright leaks) It is most likely due to air leakage from house to attic:
- Ceiling to attic is leaking moist air, condensing. Solution: air
sealing of all penetrations (light fixtures, ducting, electrical,
gaps in construction)
- Insufficient insulation, combined with air leakage allows
condensation to occur. Vapor barrier (VB) insulation (batt with
paper facing) should NOT be placed OVER any existing insulation,
creating a trap. If used, the VB is placed against the heated side
in winter. Do not use poly for a vapor barrier. The assembly must
be able to dry in either direction, should condensation occur.
Update RE can lights
Recessed cans are notorious for leakage. If they are IC rated (designed for direct insulation contact) they can be air sealed with a ASTME 136 rated firecaulk directly. If they are non-IC, a XPS box can be built that meets the clearance requirements of the fixture (insulation must be a minimum of 3" around a non-IC fixture (NEC 410.66)) Many commercial products exist for covering cans.
Video: Airsealing recessed lighting by Green Homes America
I've seen this condition develop when a bath vent fan was dumping air directly into the attic and it saturated the insulation and it grew mold down through the drywall.
Best Answer
Why would you do wall and not ceiling? It would be wise, considering the small cost and time investment. It doesn't take much heated air in our cold winters to result in substantial moisture condensation.