Palaces

Yes, but not both on the same day. Either are doable, but I don't know what you mean by a limited window. Pick the one that interests you most.

I liked Blenheim Palace the most because the grounds are so beautiful and it is out in the country. But either one would be an awesome day.

Windsor. Even though it’s not a palace, it is the largest and oldest inhabited castle in the World, and its origins are 700 years older than Blenheim.

I enjoyed Windsor, but you'd need to plan because it requires a timed ticket. You'd need a couple hours to do it justice.

I guess it might depend on where you will be. Windsor and Woodstock are not adjacent.

Don't understand about your window.

They are very different - perhaps your preference would help decide.