Most people like the upstairs on the 747. If you do, look for the exit row with lots of extra leg room.

Since demand for the upsairs is higher than downstairs, the chances of having an empty seat next to you (and larger bins--but only in the old J) is greater downstairs. The B cabin is smaller than the C cabin but many aircraft do not have the C cabin. If you fly more than once a month, be advised that radiation exposure is higher in the upstairs cabin.