I have extensive experience with both. One is not necessarily better than the other, just different. osCommerce is a better choice if you are an experienced developer who doesn't mind hacking in to the code. Mods are a little more difficult as are design changes.
Zen Cart is a lot easier to use if you are not a code person. Mods tend to be easier and the template override system works very well and is fairly easy to grasp. I sell Zen Cart installs because the admin is a lot cleaner and easier to use for the client.