This is an eternal question and we have 2 schools ; Some sensitive persons will certainly prefer the galley up, open on the salon ; less effect on the seasickness, you share life on board, you have a quick access to cockit and everyone will take part to the kitchen ; but there is offently less space and less storage, this takes place of a well appreciated lounging area, and this invites you to have a perfect clean kitchen ; then this shares also the different noises and smelles, sometimes great sometimes not really ! Well the galley up is a must for small catamarans or really for very large units.
The galley down hull is usually quite generous, with a lot of storage and fullfill the crews expectations who can work in peace and prepare carefully ; even if it's the big mess there, the salon remains a friendly space between sofas, bar, lounge ... and offently the crew has his quarters very close to the kitchen and can escape : a mutual respect which leads to a good atmosphere on board ... One disadvantage is that the galley may occupy an area where we could put an extra cabin !
So this is your work to position at the right place the cursor !