These finishes are fragile, so from a practical standpoint silver is the best choice. I like black phones though, so I’d go with black. Personally, when I’ve had colored phones, I’ve always tired of the color pretty quickly.
I bought an 18PM Pitaka and it fits fine on my 17PM. They include little foam stickers now to help with fitment, but I am not sure they are necessary. I used them just because the 17PM plateau is thinner so I thought why not. I will try to get some pics later.