Two of the most popular attractions in England are Oxford and Cambridge. If you have limited time and can visit only one of them, then well, flip a coin. If still you want my opinion- Visit Cambridge.
Travel between Oxford and Cambridge takes about 3 hours by bus X5 that runs frequently on all days of the week. It connects these two towns famous all over the world for their history in educational excellence.
The journey is quite pleasant as are the buses.
Milton Keynes falls almost midway between these two places. Do remember that you will be walking a lot at both Cambridge and Oxford. Both places are best enjoyed on foot.