Just got a chance to bring out the Zephyr for the first time this season, so I thought I'd relay my experiences with each one while it's fresh in my head. Again, I'm 6'3", 200lbs, size 14, +18/0 goofy stance.
Keep in mind while you're reading this, I fly a Titan TX 161 (26.3cm waist) and a Zephyr 166 wide (28cm waist), and I've got no doubt that difference in length and width affected some of the outcomes. I also use Burton elevators with my Titan and do not with my Zephyr.
The Zephyr is a more forgiving board than the TX, but it's definitely not as agile. Also, I tried to pay more attention to the speed factor with the Zephyr. The TX is definitely the faster board, but the Zephyr is nothing to sneeze at either. Unless you're looking to break records or enter official races, the Zephyr will absolutely be fast enough for you.
The Zephyr got a bit squirrely on me at really high speeds in a few places, but it wasn't anything I couldn't recover from on the fly, and it certainly wasn't major crash-inducing wobble. However, if I had hit the wrong kind of bump at the wrong time on the Zephyr and caught an edge during a squirrel-out, I might have gone flying. All in all, the fact that I didn't go flying in some of those spots is a testament to the Zephyr's more forgiving build. If I were to have gone through those motions on my Titan, it probably would have laughed at me and then used me for impact protection as it bounced me off the slope side until I was its bitch.
The Titan can definitely hold a better line at speed. Just like NS says, you've never experienced a faster, smoother, and more responsive board than the Titan... you just gotta keep the bitch on a leash or she'll put you on your ass.
The Zephyr is an easier board to land when you get air. It came down with a solid 'whomp' and kept going straight as an arrow, whereas the Titan would sometimes wobble for an instant when I landed and I'd have to adjust. This could be due to the extreme differences in width.
The carving trophy goes to the Zephyr, no question (again, keep in mind, 166 vs. 161). If I saw something shiney over there and I wanted to head in that direction, the Zephyr took me there, no matter how tight a turn I asked it to do. It was't as agile (read: multiple quick transitions) as the Titan, but you'd never have guessed you were riding a 28cm wide board either.
At low speeds, when glide-skating from the lift to the top of a run, the Zephyr was easier to control. I sometimes have issues with my board kind of rotating however it wants to while I'm skating long distances, but the Zephyr was much easier to handle in this regard.
At low speeds with both feet in (like hanging with my wife on the novice hill), the Titan was easier to control, but again, size differences surely came into play here.
So basically you've gotta ask yourself if you can handle the Titan (Honestly, I'm still wrestling with the bitch in spots). I think it's a better high-speed freeride board if you have the sack to keep it under control. It asks a lot but gives a lot. The Zephyr is more forgiving and holds a better tight carve (it was also longer, tho). If you're the kind of rider who likes speed, can bend a board to his will and doesn't need its forgiveness, you probably want to hit up the Titan. I still benefit from a board showing me a modicum of compassion, personally... I'm not the double black diamond type yet.
At the end of the day, however, you're going to get a big wide board chubby in your pants no matter which you get.. I promise. These are both superb sticks that deserve every bit of the good rep they have.