Zen Cart is a fork of osCommerce that adds all the features and security osCommerce it lacking (although Zen Cart is not invulnerable). If you want to use Open Source stuff, Zen Cart is as good as it gets. I don't use either anymore, I don't need the headaches.
Haven't actually used either ( though i've been through the code ), still however i don't like most commerce packages ( fixed / bland layouts, bad template systems ), if possible find a package that best suits your needs and then have a developer make any custom changes that needs to be done. Good luck in your search though.
Zen Cart will only hold up if you run it on a dedicated server or a shared server with only a few account on it... especially if you have a busy store. This may change if they ever release v1.4, but I would not spend a lot of time on the current code because it is just not a good base to invest a lot of time or money with.
You would be better off looking at CS-Cart or even a ASP.Net solution.