Depends on what you are looking for. I am primarily on Zen, but I don't sell anything. It is strictly a vanity site. I enjoy the easy access to my images when I am out an want to show someone something that I shot. The app is o.k. , the mobile site is functional, and the main system is pretty good.
However, if page views are what you want, then zenfolio is not as good as flickr. Flickr has a developed sharing, community, type thing going. You can post your pics in other groups to increase your exposure. You can friend people and then they see your photos on their feed. I guess the best way to compare them is to say that zenfolio is a stand alone store front, people can find you if they look for you, or maybe some stumble in... however flickr is like a store in a busy mall. In many cases you are accessible by nearly everyone who walks by, and the odds of someone walking in just to look around are significantly higher.
The primary reason I am on zenfolio is that I like my domain name, and I used to manage my own galleries on my own webserver. When I decided that was too much work, I pointed it to zenfolio. From a professional service point of view, if you are selling, you almost must be on something other than flickr. It is a cleaner, no ads, all about you, and your images presentation.