Do not buy tickets in the lower level (100's). Site lines are terrible as they are just too low. Also, the closer you are to the 50yd line, the further away your seats are from the field. Try to find a happy medium and sit somewhere in the 200-300 level around the 20yd line toward the home plate side to avoid foul pole obstructed views.
I don't think you have a lot of choice, other than choosing which price level you want (which determines how far up you'll be). Sounds like the ticket office will be assigning seats based on your priority points.