So this is just my opinion, but in a place like Venice I would not book the pre. With the pre, you're on Viking's schedule if you are depending on them for transportation.
In Venice, our experience was, if you want to get photos anywhere near comparable to the beautiful photos you posted here recently, you will have to be on your own timeline and that will mean being out there at first light before the hoards of tourists. I would definitely do Venice on your own. It's very easy.