The answer of this question lies in the fact that in
benzylic carbanions 6 electrons are delocalized over six carbon atoms while in cyclopentadienyl anion 6 electrons are delocalized over 5 carbon atoms , so the dispersal is more in former. We know that greater the dispersal, more is the stability, hence Benzylic carbanion is more stable than
cyclopentadienyl anion.