Groups can be used as masks, too.
Generally, I think vectors are more flexible than bitmaps; they can be freely resized without concerns about resolution. However, an advantage to bitmaps might be the ability to use the Brush tool or any of the other bitmap tools to edit the mask.
It really depends on the materials you're working with. If you have a bitmap that you want to use as a mask, then that's what you use. If you have a vector object, then that's what you use. Having the flexibility to use either is more significant to me than the advantages or disadvantages of one versus the other.