I can't answer the later question, although I suspect it will be on the day, and ultimately the balance of being delayed or cancelled or arrive early is up to you. What I would consider is the what-ifs, if your flight is cancelled, what are the load factors on subsequent flights, other means to get to GLA, and also think what you could do to make arriving early enjoyable? I for example love to shop in Glasgow and there are some fantastic restaurants for lunch.

Good luck.