I have both and absolutely love them (mine are the GMs) but there's a few things you have to consider about each.
-Yes the galliera has the plate that will get scuffed up with time, but the stresa has the "buckle" that will definitely get scuffed up too. I notice that if I don't buckle mine the bottom part bumps against it as I walk (probably wearing away the coating).
-I find they are both relatively heavy bags, but I think the stresa is a bit more comfortable on the shoulder. However the galliera is a lot easier to get your stuff out of quickly. (I usually just buckle my stresa and leave it unzipped but it's still kind of tough to grab larger things out.)
-The galliera is a hobo style so the slouchiness is great, while the stresa doesn't hold it's size as well (I noticed mine caved in a lot...but I got the jumbo size purse to go organizer and now it holds the shape).
-The galliera has had issues with the rings causing black marks (my first one left horrible marks---it was made in 2008 though---and they actually just had to replace it. I haven't carried my new one long enough to see if the marks issue is resolved, but it is something to consider.
-the galliera is everywhere. it's a great bag and i love it to death but you see it all the time. i have yet to see someone else with my stresa in person.
-also i like the interior of the stresa better. the galliera has better pockets and the nicer lining quality, but the stresa is defnitely easier to clean and doesn't show wear as quickly.
I think that covers the majority of the differences i've noticed between mine. They are both absolutely WONDERFUL bags though and i'm sure you will LOVE either one! If you have anymore questions let me know...but they are truly great bags!!