Having your own mask is always a great idea for the reason you stated and it doesn't really take up much room. With that in mind, I would still rather use a rental mask than rental fins. I'd take all my gear, but if I could only take one thing, that would be my fins.
I'm with Walter, though even more so.
I bring anything that is breathed through, looked through, peed in, or kicked with.
:cool2: