I have the GAN 13 (UV), GAN 11 (frosted), and GAN Mini (UV). My favourite is actually the 53mm cube because of its size, even for two-handed, but that's probably just me. I also slightly prefer the UV coating, but I'm happy with the frosted finish too, it's not a big deal to me.
Between the standard 56mm cubes, I prefer the GAN 11 because it has a lighter turning feel. There is a small difference in weight, but I think the big difference is that more force is needed to start a turn on the GAN 13. The corner-to-core and edge-to-core magnets reeeally want the layers to stay aligned, so you have to push through it. I guess these magnets also help you finish the turn and re-align the layers, but I don't think it's necessary to the extent that the GAN 13 does. The corner-edge magnets already keep the layers aligned quite well, and the corner-cutting is very good too. I removed the corner stalks from the GAN 11 once though, and didn't like how it felt less stable, so I think the corner-to-core magnets are probably worthwhile. But the edge-to-core magnets introduced in the GAN 13 seem to be a step in the wrong direction.
I'm curious about trying the GAN 356 M or GAN 356 XS, just to see the progression. Looks like the customization features improved over the years, but what else can be improved? Surely they can't add more magnets...