Which of these carbocations are more stable?
As we know (to some extent), resonance in a molecule occurs due to sideways overlapping of p-π orbitals.

Now in the molecule B, there is high steric hinderance near the carbocation due to which the plane of the "carbon with carbocation" changes, so that it can feel minimum repulsion from the other groups. This forces the carbocation to remain in the mutually perpendicular plane, which inhibits the resonance. 

Thus, this makes A (the less hindered carbocation) more stable than B.