The official webpage for the X5 is at https://www.stagecoachbus.com/promos-and-offers/east/stagecoach-x5 The coaches are fairly nice but going the whole varsity line by coach just glued to one seat can be a bit boring: an hour to Buckingham, then another hour to Milton Keynes, then another hour to St Neots, and yet a further hour to Cambridge. Most trips take about 3:50.
A train connection from Oxford to Cambridge will take between 2:30 and 2:50 including the connection across London. About an hour to London Paddington, about 20 minutes on the Circle Line in London and about 50 minutes from Kings Cross or St Pancras (trains from both, time of day will dictate which to take). More or less during the day depending on train schedules. Or you may catch a train on Chiltern Railways from Oxford into London Marylebone, again depending on the time.
So the train is faster, gives you a bit of London, avoids traffic, is a bit more complex, and to my mind is more fun and more scenic.
The coach - although with all the stops it makes and after 4 hours on it, it will seem more and more like a bus, even if you do get wifi and a plug point - is likely cheaper, slower, and provides views both of market towns and highway.
Where will you be in Oxford, and where do you need to get to in Cambridge?
You pays your money and takes your choice...