it depends, what you prefer. Transatlantic crossing are wonderful, if you want to relax, only a few ports of call, no stress. We have done two TA crossings, one on the Splendour and one last november on the Voyager and we loved them both.
Here is the link to the Splendour-review: http://www.oppermann-wuppertal.de/TA1/tae_review1.htm
The Voyager review will be published in english within the next days.
Panamacanal is totally different, you will see wonderful nature, the great Panamacanal, the climate is totally different, so its up to you, what you prefer. Thats the same with these two ships, we know both, have been on the Brilliance on a Caribbean cruise and on the Voyager for the TA. Both are wonderful ships, but Voyager offers more for many days at sea.
Good luck with your decision