We just went through the canal this past January and I would reccommend January. It was warm, probably low 90's but bearable. Our crew had warned us how hot it would be but we really didn't think it was that bad at all. I would think that April would be very hot and the sun would be much more intense. Also, January is in the middle of the dry season so it is not quite as humid.
We came down the west coast of Mexico from Los Angeles and it was cool until we got to Cabo San Lucas. After that, the temperature was very nice and warm.