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.
The TL;DR -- vapor barriers belong on the outside only
You should only place a vapor barrier on the exterior side of the exterior wall, allowing the assembly to dry back to the inside if water does get in past the tub surround. I would recommend paperless drywall over cement board for the tub surround, as well -- gypsum board of all types is vapor open (around 50 perms) while serving as an air barrier, allowing drying to the inside without letting a ton of humid air into the wall cavity, while cement board is a fairly severe vapor retarder at 4-8 perms, which puts more pressure on the ability of your tub surround to keep water out of the wall to begin with.
What happens when you try to stick one on the inside?
When you stick a vapor barrier on the inside of a building, all will seem well at first, until you go and turn the air conditioner on. Then, the moisture in the warm, humid outside air that fills the wall cavity will start to condense on the cold exterior surface of the vapor barrier (assuming an insulated assembly), and you get mold as a result.
A similar problem will happen if you try to insert two vapor barriers in the same assembly -- the inside one will still generate condensation despite the efforts of the outside one to keep all that humidity at bay, and now there's utterly nowhere for the moisture to go!
Of course, there's no point in putting a vapor barrier on an interior wall -- the airspace of a partition should never communicate with that of an outside wall!
More info on why vapor barriers are like badgers can be found in BSI-073, aka "Macbeth Does Vapor Barriers."
Popsicles are for sucking, bathrooms aren't
One other mistake that can be made in a modern, tight house is to slap a "fart fan" in the bathroom and call bathroom ventilation done. Doing that just means that your makeup air will come from all sorts of random leaks that have no business providing indoor air to people -- especially if one of them happens to be the range hood, or the exhaust of some appliance.
The easiest way to solve this problem is to provide transfer air to the bathroom from a zone that's being blown on, say the basement, if your basement is a suitable transfer air supply, as not all are! (You'll need to provide, oh, 10% extra makeup air as this is a transfer air setup, as well.)
Of course, if the OP's house leaks more air than a SR-71 on the ground leaks JP-7, an old-fashioned bathroom exhaust fan like the one the OP proposes will serve him fine.
More on ventilation in general can be found in BSI-070, "First Deal with the Manure and Then Don't Suck", by the way.
Best Answer
I'd use unfaced fiberglass, at least if each wall cavity has a place above the shower where the wall can breathe. Fiberglass is very permeable to air, which is why it only works well in still air.