Your city engineering/building department should have standard plans for concrete and CMU retaining walls. Pick up a set of those to get a better idea of costs.

I’d lean toward the Keystone-type block wall myself. Both the concrete and CMU walls will start to look shabby with time due to the effects of water (mold/algae, efflorescence, etc.) Weep holes get plugged, superficial cracks freak out potential buyers and neighbors, etc. And my geotechnical engineering profs spoke highly of Keystone-type walls for various other techical reasons. (Of course in those days a big pile of precast blocks was also handy for fighting off cave bears and marauding Neanderthals.)