1 - redeeming for VS award flights seems broadly similar, though CO only seem to offer 2 UC awards on each flight, even when VS has more. AFAIK there is no way to upgrade a VS flight using CO miles. One of the best features of FC is that you can UG from PE to UC at the airport without capacity controls (if there is a seat in UC open they will let you UG).

2 - If you travel domestically in the US, then CO. CO silvers get free domestic F UGs (also on NW for a little while longer); depending on the route, day of the week and time of day these are more or less easy to clear, but many CO silvers report high % success when avoiding popular flights.

3 - VS status is not much use if you fly paid UC, and CO status is probably much more useful to a US resident than VS status. Also, CO is moving to *A, so assuming the VS/CO codeshares survive that, so you should consider the usefulness of *A status in your calculations.