Well, does moisture only go into your shower and no place else? Can you securely trap it there? I dont think so, so I would use it all through out the bathroom, to prevent humidity from penetrating the walls into other rooms, or compromising other things. (In my neck of the woods, green board is only a buck more than gypsum board, or dry wall, so the cost is negligable).
(Its what I did, and its only my humble opinion).
Venting to the ATTIC? NOOOO WAY!
You will have mold up there!
You will create ice conditions in the winter time, thus promoting ROT and Decay!
You are inviting TERMITES to your location by doing this!
Who knows about other critters
No, you either vent it out the wall, or out an vent through the roof!
Personally, I would put a vent near the floor, about 12" above it, and in a location where you will not have any obstructions, that being a table, or shelf for this and that, that women love to put in thier bathrooms
(You WILL need to clean the fan blades every 6 months or a year). Near enough to the toilet to expel odors from it, and water vapors are heaviest near the floor, so they will be taken out rather quickly that way.
(No, I have nothing personally against the fan being in the ceiling of the shower, I just like mine next to the floor where it does the best of both worlds and a whole lot of good).
I vented mine out the side of the house, and it came with a hood over it, that has a spring mounted lever so that air infiltration didnt come back and cold with it. Also, make sure its "big enough"... (You dont have to get a huge 8 inch one like I did, but it has to exchange the air enough times to expel the moisture, so incase you DO go with drywall (*cringe*), then you might have a chance of saving it from damage from moisture.
Just my two cents for what its worth and a wee bit extra for the collection plate...