I've sat all over Petco Park over the years so I know where the best seats are. Section 315 aren't the best seats but for the money, our experience was great. If you look straight ahead you've got the scoreboard in left center field, the pitcher's mound down below. These seats also overlook the Padres dugout down near first base.

The Western Metal Supply Building in left field to me, is the neatest feature at this ballpark. The organization incorporated it into Petco Park the same way the Orioles did the B&O Warehouse into Camden Yards.
The Padres team store is on the first floor of the Western Metal Building and you can even rent out the roof for a game if you have a large group of fans.

Just to your right will be a set of 'Tower Lofts' which is private seating so there are no immediate bleachers to the right of these seats. I would personally avoid any row in the "20s" as those seats are just too close to the top of the section and will make your trip to the concessions/restroom s a small journey.
Every seat in the upper deck as a cup holder but the rows are very narrow and have a ton of seats in each row. If you want comfort and an up close view of the game, I would recommend the Premier Club seats. Those seats are affordable for the most part and are right behind home plate!