word choice - "an average of" vs "on average"

I am trying to write a job application with a sentence about my averaged evaluation. But, I don't know which expression is correct or more appropriate in the formal letter.

1) The overall performance evaluation is an average of 5 out of 6 since the fall 2011.

2) The overall performance evaluation is a 5 out of 6 on average since the fall 2011.

Thank you so much for your help in advance.