Cairnryan to Glasgow: bus or train?

Till somebody with personal knowledge of that route chimes in, I'll give you my general opinion.

I would nearly always choose train over bus for comfort, if both would work for my trip.

The advantage of the train: more comfortable, more space to walk around, bathroom.

If the choices are direct train vs. direct bus, I would take the train.

If the choice is between direct bus and train with multiple connections, I would probably take the bus if it’s less than 3 hours.

The other thing to consider is the location of the Glasgow train station vs bus stations in relation to where you are staying.

While I agree with the earlier comments (and would usually choose the train), I'd say the comfort factor is less important if the trip is rather short, and inter-city buses generally have nice seats in any case. Another point is that if you're traveling with larger-than-carryon-size suitcases, you may appreciate being able to stuff them into the luggage hold below the bus rather than dragging them onto the train and (probably) lifting them onto a rack.