Upstairs: I like to fill the side-bins with all the stuff I use during the flight, I get annoyed when people are jumping up every 5 minutes to get something out of the over-head locker.

However, I don't like to be too far towards the front, all the traffic to/from the flightdeck and the toliet can become very annoying.

I also think upstairs is quieter and people tend to sleep more upstairs. Another point is that because the windows are not as accessable upstairs (ie. the view is obscured) people don't tend to wake you up at 5.30 in the morning opening their window blinds.

I agree that I think the airlines think that everyone likes to be upstairs - I think some of the airlines specifically choose not to put children and large groups upstairs so it keep the noise down.

One more benefit of upstairs is that you do not get the 'tourists' from Economy just having a wander 'round.

One negative point, if the toilet fails upstairs or someone decides to go in there and read 'War and Peace' you are stuffed!

Of course with BA it is not an issue! You go up the pointy end, into the calm of the 'purple' zone and that is that.
MF