We have limited time in London and can do 1 day trip. Should it be to Windsor or Cambridge?
Depends on what you are interested in! Windsor is all about the massive castle and the Royal Family - the tour of the castle is great and there is lots to see. Cambridge is all about the colleges, punting, the history and the beautiful architecture. Both are well worth your time!
Anita's right - they are two very different places. That said, let me note that if you go to Windsor, you can tour the castle in the morning, have a lovely pub lunch just outside the castle walls, stroll through the wee little village and then continue on rambling (it'a only about a mile) until you cross the Thames and are in Eton! As in Eton College! The Eton High Street is utterly charming, and the college is gorgeous - dreamy Tudor spires, etc.
So Windsor is a bit of a two-fer. I recommend stopping for a pint at the Crown and Cushion Inn on Eton High Street. Also recommend New & Lingwood for window shopping - the gentlemen's accessories are wild!