If time is very important, it's a no-brainer, getting to NY 1 hour and 10 minutes before you left London.
Many Concorde pax. do just that, then fly BA 1st class back, hence BA's commitment to the aircraft.
Pax. starting from JFK, can get to LHR by 17:15
If you don't really like flying much, like me, again a no-brainer, Concorde all the way. But I've only ever been in Club, never 1st class, so cannot really know what 1st class is like.
But Mach 2 at 60,000 ft is always an amazing trip, far removed from the usual flying experience, wherever on a wide-body you are seated.