Although it looks like it from the website, the price of an upper level seat and a lower level seat are the same. The only advantage of picking a lower level seat is that you would be guaranteed a seat on the lower level. (This would be good for example if you were in a wheelchair or could not climb the stairs.) But otherwise, even if you chose an upper level seat, you would have better than a 95% chance of sitting on the lower level if you wanted to.

But as jackal said, I personally would spend as much time as possible in the Sightseer Lounge! ^