Ortiz had a great year, and is definitely deserving of recognition(infact, he might end up winning the award anyway), but there were three main reasons why I picked Prior over Ortiz:

1.Prior's ERA(2.43) is significantly better than Ortiz's ERA(3.81).

2.Prior had 3 complete games, whereas Ortiz had just 1, and Prior went deeper into ballgames, averaging 7.033 innings per start, to Ortiz's 6.235 average. A big reason behind this was Ortiz's high pitch count totals; he walked a league leading 102 batters.

3.Prior pitched spectacularly down the stretch, when his team needed him, posting a 0.69 ERA in August and a 2.27 ERA in September. In that stretch, he had just one loss and zero no decisions, with 10 wins. Ortiz, however, slumped in the second half of the year, posting a 4.19 ERA in August and a 4.50 ERA in September.