I've had an S6F for about a month and demoed the ZG in Cross Mountain and in Golden this spring. (I use the smallest sizes of both). You really should demo both boats. I know a LOT of people absolutely love the ZG, but I really didn't like it myself, and I know a few folks who bought them who then didn't like the boat as much as they thought they would. What I didn't like is that it felt very unstable on edge for me,... I have not tipped over this much in a boat since I can remember. Plus it felt long for our super shallow playspots.
The S6F feels like it has training wheels. Extremely stable on edge and amazingly well balanced front to back for controlled ends. Very forgiving in a hole- I've made edging mistakes that I know I would have flushed in other boats, and I simply flattened the hull and slid right back where I wanted to be. It cartwheels and loops well. I had it on the Middle Fork for 8 days and it handled pushy water better than my pocket rocket did- holds its line nicely through rapids. It is also the easiest boat I've ever been in for flatwater tricks. Obviously I love the S6F but again, there are a whole lot of people who love their ZGs.