I believe the OP only wants to visit a bullring, not go to a bullfight. However, there are a couple fights on Easter Sunday in Sevilla. There are fights in other towns throughout April. Link to the bullfight schedule. One thing about bullfights, you either like them or hate them. Few people are in the middle.
The rings will both be pretty much the same, so it really wouldn’t make much of a difference which you went to. The best bullring tour I went on was in Pamplona as one got to visit some locations (medical facility, chapel, etc.) that many tours don’t allow be to visited. The ring in Ronda is widely considered to be the oldest in Spain, constructed between 1780-1785, but Sevilla also claim’s its is the oldest.
If only interested in a bullfight museum, the one in Córdoba was very good and not overly large, so didn’t take forever to see. It does have some free entrance house.