Well, i can't speak for Marcus, but i can tell you the Leviathan mockups are done in Illustrator.
I originally traced the body and neck outlines from photographs and made flat 2D schematics of them in Illustrator for the LGM site. Then i had some finish ideas, so i added some shading and dimension to various parts in Illustrator and then took them into Photoshop to apply various effects and images.
When my Photoshop file got to over 250MB with all the layers (i was working at 150ppi), it was getting really slow, so i decided to bail on that file and go back to a pure Illustrator approach. As a result, some of the newer mockups have a very slightly different 'look' to them... Illustrator's web export capability isn't quite as sharp as Photoshop's, so the newer images are a little 'softer' in parts. Some minor finesse is missing from the Illustrator versions as well (no slight bevels around the body or cavities, for example) but the file is way smaller and easier to work with, and if i want to, i can print them as large as the photographic textures will allow.