Day trip from London to Cambridge or Oxford?

They are both worthy destinations. You would enjoy either one, but a day trip doesn't provide a lot of time. A consideration for me would be the ease of transportation. Where are you staying in London? How easily would you be able to get to the appropriate train stations--Marylebone or Paddington for Oxford; Liverpool Street, Kings Cross or St. Pancras for Cambridge? Time spent forgetting to and from the London train station will be time you will not have at your destination.

You can check schedules and fares at nationalrail.co.uk. Average train time to Oxford is a bit shorter, but for both destinations there's considerable variation from train to train.

Staying in Islington a number of years ago, took a train from Paddington to Oxford and had an excellent time during day a trip to the city. Highlights were New College (huh, found 1379) Bodleian Library, Radcliffe Camera, Mob Quad at Merton College and tasty pub lunch at Turf Tavern. Talk about grand and classic, that place oozes a learned and detailed history. I felt smarter just for having visited.

This is a tough decision. I thought Cambridge was a prettier town. But you won't regret either choice.

Both are good. Cambridge is more visually impressive. If you saw the great movie, Chariots of Fire, you might prefer Cambridge.

I can't speak on Oxford, but Cambridge is beautiful! We did a day trip there in April and it was a highlight of the trip. In particular, we took a punting tour with Scudamore's and it's one of our favorite memories of the whole trip. Highly recommend it!

We went to a service at Kings College Chapel where their choir was singing, took our punting tour, stopped by the Eagle pub, popped into the market and thoroughly enjoyed our day.

PQ

Agree that both cities are wonderful. Yes, Cambridge is more scenic and quaint...but Oxford is wonderful, too. The Oxford TI has some good walking tours, and if you are an Inspector Morse, Lewis, Endeavor fan, then you can take a walking tour of many of the colleges and pubs that are featured in those PBS series. (available through Walking Tours of Oxford). There are quite a few Harry Potter filming spots in Oxford, such as Christ Church College and New College, also. I am more familiar with Oxford, having visited several times over the years. I need to return to Cambridge and do it justice!