For me the North one felt more cozy though quite busy on arrival (12 lunch time on a Thursday) and it had more individual seats with tables next to them rather than the bench seating of the South Galleries.
The 'outside' space was a nice feature, though too busy when I was there to get a seat.
There was exactly the same catering offering at both but more newspapers at Galleries South.
The quietest area of the South lounge was next to the cinema, but the sound was turned up very loud (even though no one was watching anything).
There did seem more staff in the south terminal when I wanted them to get some champagne.

On reflection there isn't much in it. It was my first T5 flight so I wanted to try both, otherwise I would just go to the closest lounge to either the Gate, or if not announced yet then the check in.