Hmmm... That's a toughie. The Airfix kit is way easier to build, but has more inaccuracies. The CA kit is a better representation of a B-57, but is really fussy to build. I noticed the Airfix B-57 has strange bulged forward fuselage sides that I think is wrong, and the cockpit interior is pure fiction. Airfix got lazy and used the same panel lines as the Canberra, but the B-57's panel lines differed quite a bit, especially the wings. There are a host of other little problems, like the mis-shapen tip tanks, undersized FLIR pod for the G version, missing wing guns, horizontal stubs for the stabilizers, and of course the inaccurate scribed rudder. On the positive side, it has an optional position bomb bay, separate flaps and speed brakes, and the small oil cooler inlets under the intakes have better definition.
The CA B-57, like all their Canberra kits, will require a motor tool with a big grinding bit to thin down the trailing edges of all flying surfaces, although I think the rudder will need less work than the other Canberras. The cockpit tub, canopy sill, and forward fuselage are kind of tricky to get lined up. On the plus side, unlike the other CA Canberra kits, the B-57 has correct sized tip tanks. The rear fuselage and wings look the same as the other Canberras, but CA tooled entirely new ones for the B-57.
If you are not really picky about absolute accuracy and just want a cool looking B-57 on your shelf, the Airfix kit is the one to choose. If you want a more accurate model and don't mind building a limited run kit, CA is the way to go.
If only Tamiya, Hasegawa, or Revell did a 1/48 B-57...