"This is she." Is the correct usage. This isn't a matter of what is common, sounds better, or what you happen to use. Right and wrong are still two very different things and there are no grey areas in the rules of proper grammar. If everyone went around robbing banks we still wouldn't grant them the allowance just because it's "common". Languages were set up to make some sort of logical sense and follow a set of rules. When you stop following those rules you are no longer speaking that language, but instead, a warped derivative. You may use whichever method you wish, but know still that the only correct way is the right way and that way is "This is she."