For several reasons, if its a choice you have, go for Blackpool, the again if possible do both. I'm a 15 year plus KSP season ticket holder. It's close to my house so an obvious choice and excellent value.
10 years ago, I would 100% say KSP over Blackpool, not now. Neither are bad, one in my opinion has stayed a good zoo, the other lost its mojo a bit.
Also, tbh, if its a day out then Blackpool offers a bit more non zoo stuff than Prescott does! ie fish & chips, a beach and a donkey ride
Have a nice day out.