Pena should get 450-500 AB's (Kearns sucks, Griffey will get hurt or traded). Somehow someway you have to get a guy with 40 HR potential in the lineup.
Pena should outproduce Burrell in HR but not in batting average. Most of you have it backwards. Pena has more power than Burrell, but K's a bit more and lacks plate discipline. Pena is capable of swiping some bags if you're in 5X5.
In the end, with my assumption that Kearns will continue to suck or get hurt, Griffey will get hurt or traded:
Pena wins in HR, SB
Burrell wins in AVG
Runs and RBI about a wash.