I love mineral wool insulation. I use it in walls 100% of the time. Unfortunately, it's a LOT more expensive than blown-in, so that answers your question as to why blown-in. Blown-in is also much quicker/easier to install over a large, flat area.
If you have adequate soffit and roof vents, then I don't see why this has to be super complicated and expensive. It's a standard installation.
Install your lower vent baffles [YouTube video], hang your ceiling of choice, go up in the attic and blow in your insulation to desired depth.
I know it's messy, but it doesn't take long, is easy, and is a one-time thing. I blew in 1100 square feet to R-49 in about three or four hours. Machine rental was free with the purchase of insulation. Four hours in a respirator for me and the same for my wife down loading the hopper saved us about a thousand dollars.
What I'm still unsure of with your particular installation is whether you plan to have a flat ceiling or vaulted ceiling in the garage. Having a vaulted ceiling definitely complicates things, and I would probably use fiberglass or mineral wool batts for that instead of blown-in. Either way, the top down order of your roof/ventilation/insulation system should go:
1.) Roof deck (top)
2.) Ventilated space
3.) Baffles to separate ventilated space from insulation
4.) Insulation
5.) Ceiling
Here's such a system for a cathedral style ceiling (note this only works with a continuous ridge vent, as air is unable to pass between rafter channels to box vents):
And here's how it would look for a flat ceiling:
In the latter case, I still recommend blown-in, as the savings are significant. 1000 square feet to R-49 costs about $900 with blown-in where I'm at (in fact, this would be more like R-55 because it's cheaper to buy more bags to get volume discount), R-60 fiberglass would be a bit over $1400, R-46 mineral wool would be about $2400.
As for vapor barrier, I think it's a bit overrated when you have adequate ventilation in the attic space. Especially considering all the little holes you'll poke into the vapor barrier in the process of installing both it and the ceiling. If you install drywall on the ceiling and tape and paint it, this itself is a vapor barrier. Preventing condensation in the attic is achieved two ways: preventing warm, moist air from reaching the attic in the first place and providing for adequate ventilation to remove said warm, moist air when it inevitably makes it up there anyway.
I did not install a vapor barrier. I drywalled, taped and mudded seams, kept ceiling penetrations to the bare minimum, and spray-foamed any penetrations that were made. Attic ventilation is up to code and then some. Twenty degrees outside, 67 degrees inside, attic was bone dry. Heating bills were surprisingly low.
All that being said, don't take my word for it. I'm no building scientist or engineer. Ask your local building inspector to make sure you're doing what you're supposed to do.
