Square for square, I'd assume the polycryo is a bit lighter.
But, if that bivy floor tested mostly water proof/resistant (don't forget site selection!), I'd probably opt to keep the bivy intact and leave the polycryo at home and save the fiddle factor. With the system intact all you'd have to do is select your site, clear the sharps, and throw down your bivy; it would be a little too much for me to have to line up netting with a separate floor, to make sure the bug seal is right (otherwise, what's the point?). With the bivy intact, you could also throw that leg out to vent and not at all worry about the bug seal between net and polycryo.
Alternatively, you could just trim down the polycryo to match the bivy's foot print, shed a few grams, and still have confidence in your complete system.