we did a total canal transit (10 days) in January, and it was hot and muggy in the Canal Zone, the later in the season you go the hotter it gets, it can get oppressively hot & muggy- so much so that you cannot stay outside for extended periods of time. We did our canal trip on Princess and that cruise has been the most memorable of all of them by far.
We went last January. I would not want to go any earlier than November or later than Feburary. It was plenty warm when we were there and that was their driest and "coolest" time of year.
The crew members warned how hot and humid it was going to be as they had been through there in April and etc. But it wasn't unbearable.
I would definately reccomend a full transit. You get to see so much more on the western side. Most full transit crusies are 10 days or more. Typically 2 week cruises.
We went on Princess and loved it.