The Panama Canal cruises are sort of a special category. There are many different options, the main one being Full Transit vs. Partial Transit as noted above.
With so many things to consider, the very best advice I can give you is find a good local Travel Agent who is a CLIA Accredited Cruise Counselor (look for the agent's diploma on the wall, not just an agency sticker). Ask your friends and co-workers for a recommendation. The service is entirely free to you. ACC's are trained to find the very best cruise for you, after learning your personal situation and preferences, and work hard to get you the very best rate.
Many of us on this board have done one or the other, but few have experience of all the different options. That is why a good Travel Agent to combine all the different choices works well.
Our particular Panama Canal cruise was wonderful. You can see our review at Zenith Review