Hi Explorer - either of your choices would be good - they are both well run vessels but the significant difference besides cost, are their itineraries. HUMBOLDT spends a full 4 days at Darwin & Wolf, and no time at Isabela & Fernandina. At certain times of the year, spending a full 4 days at Darwin & Wolf is the best option as this is where the most action is, and if the whale sharks are there by May, then Darwin & Wolf offer some of the best diving on the planet. In May the AGGRESSOR offers only 2 days at Darwin & Wolf and they concentrate more time at Cabo Marshal, Cabo Douglas and Vicente Roca - north of Isablea and Fernandina, where you are most likely to dive with marine iguanas and possibly Mola Mola. These are significant itinerary differences and depending on your expectations - might influence your choice. Aggressor is offering $1000 off May 9-16 which puts it at almost the same rate to Humboldt. I would recommend you also look at GPS MASTER which has a discounted trip for May 6-13 and spends 3 days at Darwin & Wolf and also dives with the marine iguanas and mola mola. The Majestic Explorer and Galapagos Sky also have excellent itineraries with a slightly higher rate and if budget is an issue, then the new AQUA has a fabulous itinerary and has a trip at the end of May at an intro rate of $3495. Only 2 spaces left on it and having had many communications with her new owners, I have every confidence in her doing a fine job. Happy to chat with you by skype if you wish.