Don't stress too much about whether you should book with Viking or Silja, they are both good and while there certainly are differences between them I can't say that one is better than the other. In the end it is about personal preference.
Avoiding the C-cabins is a good idea, the "party crowd" tend to choose the cheapest cabins so there might be some loud drunk people in the corridors late at night. But a B-cabin or better is fine. The standard cabins have four beds as described and are not large, usually around 10 m2. But there are other options as well, including larger cabins with double beds as well as larger suites. Having a window is nice, but there are also plenty of windows on the ship as well as a sun deck so there are many opportunities to see the views.
But I'm not sure avoiding the weekend is important in July, the party crowds are more spread out in the summer in my experience. But they are also pretty easy to avoid, the ships are large.
Prebooking some food can be a good idea, no matter what option you choose. And it can save you a bit of money as well compared to paying onboard. If you don't feel like the buffet there are many other options on all ships, there are plenty of restaurants, bars, pubs and cafés.