I feel if Rory has trouble taking Penn down and it is a striking contest for the most part, then Penn will win. If Rory gets Penn down at will I think Penn has to go straight into offensive BJJ mode, which he isn't known for off his back. But he will have to be ready too.
Either way Penn will have cardio issues if he doesn't finish early. Hopefully he comes out nuts and puts it on Rory early. Probably not though.