I looked at zen cart a while back but it wasn't as feature rich as other carts. But it depends on what you need.
You may look at creloaded.com it's OSCommerce fork that has all the latest tweaks and modules installed already and it's more admin friendly. OSC is known to be slooow in updates.
The is also:
PHPShop (and mambo-phpshop)
And some others I'm sure I'm forgotting.
I know that doesn' answer your question exactly but I hope it helps.
Zen Cart and ShopSite are 2 completely different solutions...
Zen Cart is open source and php based while ShopSite is commercial and cgi based...
From a features perspective - they are both very feature rich. The major differentiators are:
1) shopsite generates static html product pages > greater SEO functionality
2) shopsite integrates with quickbooks
3) zen cart is easier and cheaper to customize
4) zen cart is more flexible with respect to product imports (if youre using the ezpopulate contribution) [shopsite also contains an import feature, but, it isnt as flexible]
These are the major differences. As a summary - if you have extra money/time and inherent SEO is important to you, shopsite is your choice. However, if you have a smaller budget, are willing to drive traffic to your site through various alternative methods and want something full featured and easy to use - zen cart may be the better choice..