This is a fight I would love to see!! Speed advantage would go to Penn (only in round 1 and probably early round 2. Than Penn fades like usual). Strength goes to Diaz, Standing goes to Penn, but Diaz has a huge reach advantage. Overall skillset I would give to Penn as well.
With all that being said however I think Diaz's reach advantage would be to much for Penn to overcome and would probably get picked apart standing. I am a big Penn fan saying this and this is just what I see happening. If it went to a grappling match, this would be fun to watch and I think Penn would have an advantage over the much bigger Diaz.