This is something I compared some time ago. For 7 years I travelled from Edinburgh to London on Monday morning, returning on Friday afternoon and I tried both options to test which was really quickest.
For me the plane won hands down every time. I caught bmi’s flight BD51 at 0630 (varied depending on season), which meant leaving home at 0520 (had security timed to perfection to get straight onto plane). The earliest I got into my office in Westminster was 0850 but usually it was 0930.
For the train I would still leave home at 0520 to catch the 0550 from Waverley to Kings Cross and that train doesn’t get into KGX until 1012 with an additional 30 mins from there to Westminster.
The return journey was almost the same, so bmi wins this one for me.