300 level seats are the "club level" type seats.
You'll have an enclosed, air conditioned walkway behind the seats whear the concession stands are. Views are pretty good from that area. The service from this grouping of seats is pretty solid. Lines are shorter because the only people on that level are 300 level seat holders.
I'm assuming the roof is going to be closed as well, so the air conditioning isn't that big of a deal.