There are at least three factors to consider... total reflectivity per unit surface area (which reflects more), how big is the reflector, and how well can you aim the reflector. Prismatic reflective tape might approach plastic reflectors in reflectivity, but how much tape do you use and, more importantly, how big an area of tape can you accurately aim behind you.

Little bits of tape on angled seat stays will not reflect as much light as an equal or smaller area of reflector that is correctly aimed back.

Big reflectors properly aimed are much more effective than bits of tape wrapped around round tubes.

Another good alternative option... reflective tape on front & rear edges of crank arms give nice motion...