I have both bags so I love them both.
The Artsy is a little more structured/a little more formal ---it looks great dressed up or dressed down. The GM holds alot too. I am saving up and thinking of buying the MM to force myself to downsize because I carry way too much stuff.
The Galleria is a little less structured, it seems to hold more. It also smooshes down more than the artsy which may make it a more casual bag. I use both for work so the smoosh effect doesn't bother me but this is just my honest opinion.
If you need a bag that is good for both dressing up or dressing down, i say go for the Artsy now and try to find a preloved Galleria so you can have both styles
They are both great bags and which ever you choose, I'm sure will look wonderful on you so good luck with your decision.