Either way, I prefer to go to Liverpool Street (although this is due to the traction!).
And the scenery is nicer, it still has the feel of a country railway line for me, which the Kings Cross route doesn't.
Back to the original thread though, I suppose that it depends on how you define "route", but I would have thought that the Brighton line was busier than Thameslink North, particularly as far as Gatwick Airport.