Most of you folks who have posted to this thread are fairly experienced divers, I think, so you have probably learned from past experience that the oceans that we dive are not an aquarium. But just in case, and for the benefit of other readers who might take what I wrote at face value, let me temper future expectations by assuring you that the sightings that I described in my replies did not happen on every single trip. In fact, the best of it happened in one trip to Cocos and one trip to the Galapagos. The Georgia Aquarium is about the only place - a pretty amazing place, I might add - where the sightings are guaranteed (minus the Mola-Mola, that is).....but I digress.
So bear with me as I give you a little of my history. My first trip to the Galapagos was in May 2001 - it was a 10-day trip and I went there with great expectations, but came home with no whalesharks, some - not a lot - of hammerheads, and didn't see, and didn't even know that there were Mola-Molas. Still had a lot of fun seeing stuff I'd never seen before both underwater and on land.
My second trip was a 12-day trip in late November of the same year. No whalesharks but saw a lot more hammerheads - sometimes over 100 in Darwin. Due to the expanded itinerary, we were able to visit and dive Punta Vicente Roca. We were told that there were mola-molas but no one had an idea of where to look. Many of us chose to bypass the 58-degree peasoup at the surface and proceeded to 110 ft. in super-clear, 53-degree waters to see the red-lipped batfish. Then back near the surface, we got to play with sea lions. And that was pretty much it for my first experience with the Icebox. Still a really fun trip with lots of new stuff seen and new islands visited.
Having struck out on the first two tries, I then chose a 7-day early October 2005 trip, figuring that it was better aligned with the prime whaleshark time. I was beyond ecstatic when we were told upon boarding the boat that there had been 15 whaleshark sightings on the trip immediately before ours, and over a dozen the one before. It was our luck, however, that the currents were reversed on our week - in fact they were outright crazy one day at Darwin: in a span of probably less than two minutes, we went from 40 ft. to 90s, back to 40, back to 90s, before finally getting out of that yoyo. The surge at the Landslide at Wolf would carry us a good 20 ft. one way and 20 ft. the other way. In the end, no whalesharks, but still decent hammerheads and still a very enjoyable trip.
Which brings me to 2007 - a truly magical year, I think - maybe because it was also a La Niña year. I did my third trip to Cocos in July that year, a 12-day charter. Over a hundred hammerheads just about anywhere you went. After the trip, I remember freeze-framing thru one scene shot at Alcyone and counted over 200 hammerheads overhead. Cocos is not known for a lot of whalesharks but on that trip some people saw 15, including a doubleheader at Dirty Rock. And of course we also had the humpback and calf resting for 3 days in Chatham Bay, at times cruising back and forth out of the bay.
In October of that year, I was very fortunate to join a 10-day charter to the Galapagos. I wrote a trip report that you can search for in this forum. In four days in the Northern Islands, there were over 30 whaleshark sightings. There was a video sequence shot by one of the DMs that showed almost 2 plus part of a third in one frame. From there, we moved down to Punta Vicente Roca, and it was in two dives there that we had the 3 mola-molas, along with the horn-shark, the seahorse, and we also got to swim with penguins. One of the mola-mola sightings was at a cleaning station at not more than 45 ft. When it was sighted, we all took cover in the rocks, and the thing just stayed there virtually motionless as it was cleaned. I was no more than 6 ft away from it, clicking away with flash and all with my point-and-shoot, and the thing remained there unphased. But, all it took was one guy swimming towards it to get a closer shot, and it was gone. And yes, Arturo, sadly for you, it was around 53 degrees.
Bottom line, especially as it relates to the original post with regards to which is better: again, sightings are not guaranteed. Regardless of the criteria for choosing one destination over the other, the results can still flip-flop back and forth because there are too many variables at play. What the above diatribe attempts to illustrate is that you can improve your chances by picking a good year and a good time of the year, and by hopefully having good guides who know where the animals are and will get you there, and who have good eyes. And then you pray a lot, because nothing beats good luck.