If you're going for the actual better seats, Section 140, Row P are better than 139, Row M. Section 140 is closer to midfield behind the visitor bench. Having said that, both locations are among the best seats in the stadium. They are at an ideal height (10-20 rows from the field) and are near the 50 yard-line. If cost isn't an issue, go with 140, otherwise you'll still be very happy saving a few dollars and sitting in Section 139.

Sections 139 and 140 at the Edward Jones Dome
Rows M and P will be similar distances from the field in Sections 139 and 140, however the seats in Section 140 will keep you closer to the great sight lines found at midfield