Option # 2 is the only one that is ABSOLUTLY NOT possible.  If your flight from Canada is ever late (and with Airtransat, that’s not hard to imagine), then you’ll miss your flight, be out the money, and still be stuck in Scotland.

Is there some reason your flying into Glasgow, and wanting to get out so quickly???  Why not give it a chance since your going to be there.  It’s such a great place to visit.  My last trip to Europe in 2005, I flew direct with Zoom into Glasgow from Halifax.  I didn’t spend much time in Glasgow either (I think I landed at 6 or 7 AM, and took a noon bus to Edinburgh).  If you use Megabus, it only costs 1 pound.  Edinburgh is so worth checking out, as is the rest of Scotland.  I spent a total of 6 days between Edinburgh and the Highlands, and then caught an Easyjet flight from Edinburgh to Amsterdam. 

If something like that doesn’t appeal to you, then of your 3 remaining options, definitly go with #1.  You’ll be tired and jetlagged as it is, so stopping, getting the chance to relax, and getting a nights sleep, is a good option.

If you did # 3 or 4, you’d be hanging out waiting for that afternoon flight, and then after the flight, your still not where you want to be, so you’d have to hop on a train and head for Brussels.

Paris to Brussels is about 1.5 hrs, and Frankfurt to Brussels is about 3.5 hrs.  You’d be getting into Brussels pretty late, and you’d be no further ahead then if you stayed in Glasgow for the night.  I think the train rides from Frankfurt or Paris would actually cost you more than the night in a Glasgow hostel.

Also, with budget flights, chances are they’ll never drop.  As the date gets closer, they’ll most likely go up.

Yeah, there is no way #2 will work, you are supposed to check in with ryanair 2 hours ahead of time and it takes an hour to get to prestwick. 
 
I think the hostel in glasgow (like all in the UK) is pretty expensive, like $30US.  But if you do decide to stay for the night, I highly suggest a visit to Paisley.  Its a small, quiet town with old streets and buildings and a river running through it, nice intro to scotland, and its closer to GLA than the city of Glasgow (The train to prestwick leaves from Glasgow city, not the airport, but then it stops in Paisley, so its actually quicker and cheaper).  At GLA just ask information about Paisley, they will give you the bus # and some tourist info. 
 
 

 
I am sorry, but I have to totally disagree on Glasgow.  It might have been know for problems in the late 70’s and 80’s, but it certainly is not now.   There has been a economic boom in the area for last few years.  It also has a very lively arts and music scene.  
 
I felt much safer in Glasgow too, for a comparison, I would rather walk around midnight in Glasgow by myself than walk around St. Jamestown in the evening in Toronto..and I went to highschool down the street from St. Jamestown.  Brussels on the other hand..I wouldn’t fly into it late at night or too early in the morning.  Brussels looks much better in pictures than in person…it’s boring and alot more run down than I expected.
 

I agree wholeheartedly.  Glasgow is a great place to visit, the music scene is incredible. Edinburgh is just an hour away, too.