cube cart is very nice you should check it out... i like it because the wide range of payment processors and other software that intergrates with it
Stay away from OSC unless you are a security expert because that program has more holes than I can keep up with. I've installed every patch on a freshly setup host, renamed directories and used htaccess to add additional password protection and in just a few days it'll get hacked. Look elsewhere.
I have only used Oscommerce but it was ideal for my very busy shopping site that was taking hundreds of orders a week. I had no security problems and alot of great contributions exist like SEF URL's etc... and all the code seemed to work quickly and efficiently even after my dodgy tampering trying to customize it. It's also free so you cant ask for so much more.
If you really want to use the oscommerce script you should consider using creloaded instead. Creloaded is based on the oscommerce script but with a lot off contributions allready installed. There is a free download available at the website creloaded.com (standard version)
That is kind of like asking which is better: getting hit by a car or hit by a truck? At least Zen-Cart has frequent updates and security patches. It includes everything built in that osCommerce is missing. I don't recommend either unless you are operating like a hobby in hopes you *might* make some money. If you are serious, stay away from them.
OsCommerce/Creloaded are a real pain when you need to upgrade those...especially if you have installed third party modifications to those as those modifications almost always are in core files. Further more..the code is a mess and bugs take ages to fix. So I would go for zen cart.
Zen cart to me is a knock of version of osCommerce. In my opinion,right out of the box Zen Cart looks super ghetto compared to osCommerce. These are just my opinions.
Well after ..running almost 300 estore on oscom ..I would definalty give them the nod... Lets just say ..i have cut that system up and added so much that it is by far really robust and is very nice to use in general. The admin panel is slick ...I do see a AJAX upgrade with this very soon as well. IMPORTANT..MAKE IT AS EASY on the end-user with there credit card check out process ..I did upgrade to a "one click" process to stream line the sales. It is A must for any ecom systems ..there really clumsy when it comes to "quick check out" for most it will be ok ..but when you do what we do in a day ..the biggest complaint was the check out system. Hope this helps..
But then again, Zen Cart has a better template override system so it's easier to make it look fantastic. Here are some examples of Zen Cart stores - http://www.kerrinhardy.com/zen-cart/zen-cart-gallery/
Another advantage to zenCart over OSCommerce is that zenCart does not require that you have register_globals on, which is a very good thing security wise.