HI!
There are definitely advantages to land based touring as well as Galapagos Cruises. Hotel-based touring allows you to experience a bit more of the local "culture" and social scene of the various inhabited Galapagos Islands and spend the days visiting other islands, but returning to your comfortable hotel afterwards.
I havenīt personally worked with Zenith, but I have booked both land trips and cruises through www.galapagosislands.com and Iīve had successful journeys each time.
The Penguin Pursuit program that we did had us visit Isabela, Bartolome, North Seymour and we stayed on Santa Cruz Island and Isabela. We got to see a little bit of everything: lava tubes, tortoises, penguins, boobies, the highlands and beaches.
Good luck and have fun!