Train vs bus to get to and from Glasgow

I'm starting the West Highland Way at the end of August and am looking into the best travel options before and after my hike. I'll be arriving by ferry in Newcastle, so will need to go from Newcastle to Glasgow, next day Glasgow to Milngavie, Fort William to Glasgow once I've finished, and Glasgow to Newcastle the day after.

The train journey from Fort William to Glasgow is supposed to be beautiful, so I'll probably do that. Glasgow to Milngavie by train seems easiest.

But how about Newcastle to Glasgow, and back again? What is best: train or bus? Does anyone have advice? And do I get tickets in advance? And where? Any suggestions/tips are welcome :-)