Bond rotation energy comparison of aromatic compounds
Cleavage of the double bond will lead to both compounds having two rings that are aromatic, however the amount of resonance energy(hence stability) will not be the same.

Both systems will have a Cyclopentadienyl anion, but one will have a Cyclopropenium cation
while the other has a Tropylium cation . As the Tropylium cation has more resonance structures, it will have a higher resonance energy and hence greater stability than the Cyclopropenium cation. So (b) has a lower bond rotation energy.