I know lots of folks avoid paying the seat reservation fee for a number of valid reasons, but if you care enough to spend time checking out what taken and what’s not, my advice would be bite the bullet, pay them, get the seats you want and move on. Select the upper deck, a window/aisle pair is nice if at least one of you wants a window. If you don’t reserve there is a very good chance you will end up an aisle seat and a middle seat next to each other in the middle section (at best).