Which gives better performance in render/viewport: mask modifier or assinging a transparent material? I have a character model where a good portion of the skin is covered by clothes. To prevent the skin from clipping through the clothes, I assign a transparent material to the areas that aren't supposed to be visible (I can't delete these areas, since the character has multiple outfits with varying lengths of sleeves/pants). I recently came up with the idea of using a mask modifier instead of a transparent shader for this purpose in order to get better performance. My question is: would this actually improve performance? I think the answer would be different in the viewport than in rendering, if this is the case, I'd like to know how it effects both.
A transparent material still costs vertex transformation, BVH calculation, and fill rate, so a mask modified mesh will have better performance.  The difference between the two will probably not be large enough to notice.
Viewport vs. render doesn't matter-- a viewport is a render-- although you'll have slightly different costs depending on rendering engine (and a solid view is workbench, a mat view, Eevee, and a rendered preview, whatever you have set.)  For example, only Cycles does BVH calculation, and fill rate is really a rasterizer (workbench or eevee) characteristic (as far as I understand it, I'm not a raytracing guru.)