Neither.

Until you structurally strengthen the joists to be capable of supporting the loads. Think at least 2" by 12" ,with a center beam (2 car wide garage). And support posts under the beam.

In other words, the attic floor will need to be built like the first floor (over a basement) of a house. And the joists should be on 12" centers under the bathroom just for added support.

Then, you can add whatever you want.

What Mac said. Plus, there are fireproofing and insulation requirements, as well as headroom requirements that make finishing out a garage almost impossible to do without rebuilding it from scratch first.

I don't think it's clear from the post that the attic isn't already structured to carry a floor and with adequate headroom. It's fairly common in some areas for the area above a garage to be built as space that can be finished at a future time.

how you going to heat/cool that room over the garage
surrounded on all sides top and bottom by extreme
attic temps?

foam sheathing on attic side of walls?
spray in foam on roofline?
these rooms are a real pita to heat/cool.

best of luck.