Acrylic and plexi aren't the same thing, plexi is much more brittle. If you bend both, the plexi will crack, the acrylic will develop a crease but stay together. If price is equal, I would get acrylic. However, at 3/4" thickness or so, I think either would be plenty, so - go with the cheaper one if there's a price difference.

Might talk to the seller as well, if they know a lot about plastic, one of them may be easier to work with down the road as far as fixing scratches, cloudiness, things like that go.