I would get the Galliera. I have the Tivoli GM and while I love it, the Galliera would've suited me better as it has a single strap and would sit on the shoulder better. Tivoli slips off my shoulder now and then, plus if you don't have much in it - it will sag a fair bit and loses some of it's shape.
Well you stated Tivoli PM and to compare it with Galliera GM, it's mighty different in size. The Tivoli PM is cute but more of a handheld bag unless you have skinny arms.
The Tivoli GM is better in comparison with the Galliera PM, I say go for the Tivoli GM if you're concerned about the closure (zip), Galliera (magnetic). I heard that a lot of people are troubled by the magnetic closure not aligned properly when things are put in the bag. But if it doesn't bother you, why not?
The Galliera GM on the other hand is huuuugeeeeee!