So five empty seats out of 20 equates to 75% occupancy, but you are in 64A (a very good seat by all accounts) and no one in 64B? I'd be tempted to stay put. If you move downstairs and you're the only one in the cabin then I'd wager that the CC will forget all about you and we'll have a thread tomorrow ranting about poor service etc. etc.
All joking aside, if it was me, I'd stay upstairs!