I'm gonna say Penn takes that one via first round KO.
Sanchez isn't a good striker. His striking works because striking in MMA sucks. He throws fast, he throws hard and he keeps throwing which overwhelms a lot of people early. Still he shows rudimentary striking technique, throwing set combinations and almost never cuts angles. Hell, even Clay Guida managed to start to figure out his striking in the third round of their fight and started countering him.
BJ Penn on the other hand, is a world class striker. He's not just some heavy handed MMA guy. From the mouth of Freddie Roach himself, Penn has the best pure boxing skills in MMA. If there is one thing Roach knows, it's boxing. And coming from someone who has trained with both Anderson Silva and Andrei Arlovski, that is a hell of a statement.
So yeah, this is an exciting match up, but if you really look at it Sanchez has no way in hell of winning. Going to be a dominating performance from the Prodigy. If Sanchez is smart he won't try his usual strategy of swarming with strikes. His best bet IMO is to try to EXPLOSIVELY get the takedown. None of that wearing him down against the cage shit, that won't work unless you're GSP. Penn's known for an airtight, methodical BJJ game while Sanchez excels in scrambles. I'd say Sanchez's only real chance is to try and create scrambles up against the cage and try and secure some dominant positioning.