We've used both osCommerce and Zen Cart for client sites. In my humble opinion, Zen Cart is a great choice for small businesses -- if it already has every feature you think you'll need as your e-commerce site grows (and if you can find a designer or template* that's compatible with the latest version of Zen Cart).

There is only one manual available for Zen Cart (visit zencart.com for more information) -- and it's a must have. Zen Cart's admin area is easier on the eyes than osC's, with more bells and whistles out-of-the-box -- but there's nothing Zen-like about its counter-intuitive admin navigational structure. Without this guidebook (or help from Zen Cart's website), you can spend hours fishing around the admin area to configure features you *know* are there. Somewhere.

If you're planning to purchase a pre-fab website layout, there are many more osCommerce commercial-qualtity templates available than there currently are for Zen Cart (1.3.7). That said, Zen Cart comes pre-installed with most of the features -- gift voucher capabilities, WYSIWYG content management, PayPal Express Checkout, multiple product images, group pricing, online "showroom" and e-commerce catalog features, optional review system that can be easily turned on/off, header tags control (for browser title bars and keywords), and lot of other little trimmings you may want -- whereas you'll likely have to pay someone to install these if you were to go with osC.

Both Zen Cart and osC have a QuickBooks Import contribution/mod that must be installed by hand, both have an order editor mod you'll also want installed (so that you can update customer information and change orders as needed); cross-sell/upsell mods are also available, but not pre-installed, with both carts.

If the ability for customers to purchase without having to create an account is an absolute must, there's an osC contribution for that -- but not for Zen Cart yet. Same goes for a Wishlist feature. The default template that comes with either cart is "uglier than a mud fence," as my Southern grandmother would say. So purchasing a commercial template or, better yet, paying an experienced designer to create a unique, branded look for your site is a must.

Bottom line: I'd recommend Zen Cart for smaller businesses, if you've looked at the features and it already has everything you must have. For larger enterprises with the budget to pay for infinite site add-ons, I'd go with osCommerce because there are hundreds (if not thousands) of contributions/mods available so there's no need to pay someone to re-invent the wheel when you need additional capabilities.

As for hosting, most inexpensive service levels will come with everything you need (e.g. PHP with MySQL) to run osC or Zen Cart -- it doesn't matter if you use Windows or Mac on your computer.

Choosing the right cart is daunting, to say the least. Hope this helps.

Scott

*Caveat emptor: All commercially-available templates for osCommerce or Zen Cart are not necessarily compatible as-is with third-party modifications or "contributions," so make sure you find a template vendor who will help you sort out the occasional template glitch that may arise from any site add-ons you choose.