This question, once upon a time, would have been comfort-focused. That would have been when I had a railcard though, and thought nothing of popping down to London for the day from Swansea.
These days, near-ish 8 years on from my last railcard, I have so much to spend money on that I have to make careful decisions about my travel. I rarely take the train anywhere, I just can't afford to. Quite, National Express get more of my money for long-distance trips than the railway. I could (if I wanted to) book a day trip to London for tomorrow on National Express and pay about £26. Such a trip on the railway would cost me far more, add in food/drink and a Travelcard and it just isn't worth it.
Not that I plan to go adventuring for a while, the money I'd sidelined for that this month has been spent on buying a new phone which has become a bit of a necessity but should save me money come the end of my contract in November. So long-term it will be worth it.
Someone mentioned comparing flight prices with comfort, especially on long trips. Well yeah I suppose I do too, to an extent. I only fly Economy, regardless of route, as I simply cannot afford to fly in any other class. It does mean I can do more trips in an average year though, so that isn't a bad thing. Yes I even flew Economy from Manchester to Sydney!
It's not the most comfortable way to go, and it wasn't with my top choice of airline, but to do Manchester to Sydney and back, via Abu Dhabi, on my choice of aircraft was just under £609. Compared with Emirates who wanted a lot more than that, BA wanting £841 and Qantas wanting something silly like £1,100, the choice to fly with Etihad was obvious. Not that I'm willing to fly with them again, but that's been covered in depth back in March 2019!
For me when it comes to flying the considerations are fares, which airline it's with, departure/arrival airports, connections to/from the airports and what craft is booked for the flights.
When it comes to trains, it primarily boils down to cost. It can be argued going via Birmingham on WMR/LNWR Advances to London is not in all that lower a level of comfort compared to GWR, and the cost saving via Birmingham can be a huge factor. Yes it's usually quicker on GWR but the longer journey via Birmingham also allows for a nice little leg stretch during the connection which can be considered a bonus.
Indeed it's fair to say that comfort on National Express is usually better than that on GWR too. I'd also generally choose National Express for a trip to Heathrow, given the journey by rail is far less convenient.
So all in all the train often loses out here!