Thank you Rick Steves' Europe for having this forum. It helps to be able to learn from all of you and decrease the stress of guessing on what is best. My question this time is, would it be better to fly out of Glasgow or Edinburg to get to London? Thanks for your help.
It really depends which of the two cities is a better fit for your Scotland itinerary. If you're planning to end up in Edinburgh, it probably doesn't make sense to shuttle over to Glasgow to catch a flight to London. That said, there's frequent bus service between the two airports. https://www.rome2rio.com/map/Glasgow-Airport-GLA/Edinburgh-Airport-EDI
There's also frequent airline service from both Scottish cities, so I would just look them up and compare schedules and fares and which London-area airport a given flight lands at. For example, I use Kayak.com to search for flights. If you filter by "Include Nearby Airports" I believe it will include both Glasgow and Edinburgh on the departure end, and both Gatwick and Heathrow on the arrival end.
Are you going to London just to catch a transatlantic flight back home, or are you going to London for sightseeing? If the latter, I wouldn't fly at all; I'd take the train. You can get to London from Edinburgh in as little as 4 hr. 20 min. and from Glasgow in as little as 4 hr. 30 min. Given the hassles of flying--extra time at the departure airport, traveling between city and airport, etc.--flying will not be faster.
I agree that unless you are catching a flight, the train may make more sense. Unlike in the U.S., trains in the UK are efficient and plentiful.
Plus you didn't mention which London airport you wanted to fly into.
To add to what epitd already suggested, you need to figure out which London airport will be the most convenient for you when you arrive. Both EDI and GLA have very frequent flights to both LHR and LGW. Easyjet even flies in to LTN (London Luton).
If you're taking the train down from Aberdeen, it is very slightly longer from Aberdeen to Glasgow as it is from Aberdeen to Edinburgh. You just have to figure out which will be more convenient - Queen Street Station to GLA, or Edinburgh Waverley to EDI.
p.s.: Are you flying on to Paris, or taking the Eurostar?
For trains > www.nationalrail.co.uk
Edinburgh > KGX or Glasgow > EUS
The cheapest advance fares for specific trains appear on the system around 11 weeks ahead. pay on the day can be expensive).
If using the trains, I would pre-book with www.virgintrains.co.uk if going from Glasgow and www.lner.co.uk of going from Edinburgh.
If going from Edinburgh - and you have time - I would break the journey with an overnight in the interesting city of York. (You would need to buy separate tickets for each leg).https://www.visityork.org