Hi T., My son is only 2, but we have lived in SP for 6 years now so I have talked to many different people about the school districts around here.
Sun Prairie schools are generally very good - living here I have neighbors who work at the schools, and neighbors whose children have gone K-12 through SP. The negative things I have heard generally are about the high school. And the new high school opens next year, which is going to be huge. The elemenary schools all have a great reputation - with a few that are regarded "better" than others (based on neighborhoods). Another thing great thing SP is adding this fall is 4K, which your son would be eligible for soon.
Cottage Grove schools also have a great reputation. The school district is combined with Monona, so the elementary schools are near home. Middle school and high school are combined, so living in Cottage Grove the HS kids have to go to Monona each day via bus.
I have heard good things about Deforest/Windsor schools - but nothing specific. And I don't know anything about Marshall or Columbia.
Hope that is somewhat helpful. Feel free to contact me via message if you need/want more info. Good luck in your move!