Have done Both spring and fall. It doesn't really make that much difference because the canal is near the equator and it is always hot there. Spring may be better because you avoid the possibility of a hurricane in the fall.
Saying what cruise line is best is impossible. That's a matter of personal preference. We prefer Princess or Holland America. If you are young and heavily into partying, you might prefer Carnival. My wife and I are recently retired so we don't. If you do not require constant partying, rock climbing etc. to be entertained, and if tropical breezes, a tropical drink, interesting companions and a good book or a little romance relax you, then Princess or HAL are both wonderful, with HAL slightly more upscale.
This is a great cruise for a balcony if you can possibly afford it. It is not that much more than an ocean view. Go to the cruise line websites and check the itineraries and the ships, deck plans etc.
Also, there are two types of canal cruises, half canal that go from a Florida port like Ft. Lauderdale go through the eastern Locks into the central lake, then turn around and come back to Florida.
the other kind goes all the way through from say L.A. to Florida or visa versa. On the later, you get the Mexican Riviera, and some of the Caribbean too as well as the canal. A few more days and a little more expensive, but worth it.
I see you are from Colorado too. Email me and I'll send you some pics from our last one on the Island Princess.
Landlocked in Denver, but cruisin every chance we get.
Polynesia, Carib. '86
Cr. Odyssey, Scandinavia, '91, 30 Day S Pac. 2002
Crystal Harm, Aust., N.Z., '94
Royal Odyssey, AK,'96
Old Cr. Pr. Canal, '97
RCCL, Carib, 1998
Volendam, Car, 2000
Ryndam, 35 day S. Am., Antarctica, '03
Is. Pr., Canal, 2004
Statendam, 34 day China, Japan, AK '06
Cr.Pr., Carib. 08
Eurodam, Atlantic, Med. '10