You want to use Magento if you are serious about designing and launching a great store. There is a bit of a learning curve but the end result is much more professional. Nigel
If I were to choose one between OSC & ZenCart, I will choose OSC because you will be able to expand the features using lots of extensions (though many requires you to modify the code) .
I had try more than 10 ecommerce system, like ZenCart, phpcart, magent, prestashop, ... I feel prestashop looks morden and faster. But I don't find affiliate program(that is very important) in any one of them.
Have a look at actinic instead, it comes opimised and is reasonably cheap for the starter edition, i think theres also a free trial version too. I looked at Zen and OSC, both were too much work to configure, but then again I'm not a developer.