Perez threw a whole bunch of innings last year, 196, with a 1.22 k/inning ratio. Peavy threw about 166 innings last year, with a slightly lower 1.04 k/inning ratio.
you have to base this decision on two things. (1)Will Perez wear down like he did last year in September? If the Pirates think so, will they keep him from pitching so many innings, therefore lowering his k total? (2)Will Peavy be healthy emough all year to get his inning total as high, or higher than Perez's, therefore making the race for strikeout's a bit closer.
Perez's ERA was a lot better than it looks on paper. He had a 4+ ERA in Sept. So dont choose Peavy thinking that you are guaranteed a better ERA.

"I live for this game. If baseballs went well with eggs, Id eat them for breakfast..."