I followed the Zone diet on and off for several years. It was an excellent springboard for me into Primal eating. I really don't think there are vast differences in the eating plan...Zone like PB suggests filling up on fat if you've met the daily carb/protein requirements and are at a healthy weight. Zone emphasizes lots of low GI veggies and fruit and no sugar. It does, however, include whole grains and, as Daemonized said, regular meals and snacks. Interestingly, I had instinctively dropped the snacks even before hearing about primal. PB and Zone both emphasize insulin control and both prefer workouts that aren't crazy cardio.
For me the biggest advantage of PB over Zone has been not worrying so much about exact breakdown of every meal. I used to enter every single meal into a planner and adjust to hit the right ratios. I was successful at weight loss and maintenance, but who wants to live with that level of obsession with food? Every time I stopped recording intake, I'd gain weight. Primal has given me easy guidelines and less to obsess about. I don't record anything anywhere, I eat well, and I'm very much at a stable point. I can imagine that people who NEED numbers to guide their eating might do better with Zone or Zone/PB mix.
Zone diet on and off for several years....worked, but too much focus on exact meal composition
Primal since July 2010...skinniest I've ever been and the least stressed about food