DH and I did a Panama Canal Cruise in April of 03.
It was very very hot (105)especially during the Canal Crossing. I don't know how much cooler it would be if you went during Jan or Feb.
On the flip side is the Mexican Coastline...Puerto Vallarta, Matazlan, and Cabo were nicer in April than when we did a Mexican Riviera Cruise a few years ago in Jan.
It was a little cool in that area while cruising. When you got off the ship in ports it was warmer.
I probably wasn't much help...maybe someone else can tell you if it would be a little cooler in the Canal area in Jan.
One thing nice about going in April is you don't have to worry as much about travel conditions due to bad weather. I don't know if that would apply to you...but being from Michigan it is usually a concern for us. January would probably be cheaper though.
Now that I probably totally confused you....Good Luck