Both are based on the same code.
I will say this for OS Commerce: If you are willing to go with the default skin, it makes for a VERY fast time to live experience, along with the security holes that come with it. Also, when you go to the OS Commerce site and look for modules and what it takes to integrate them, you'll find that better than 90% of them are not modules at all, but extremely invasive patches. It is not modular in design by ANY stretch of the imagination. Also, the database is NOT normalized at all, so you will see many tables with duplicate data. Lastly, there are many, many bugs in the application; most of which can be worked around.
If you want to customise the look and feel, close your door and be prepared for lots of cussing because that application is the poster boy good example of a bad example of PHP code. I'm using it for two ecoCode reuse? Data abstraction?
We have two OS Commerce sites we're launching in a couple of months but one of our first projects we'll be doing after going live is using the profits from those projects to fund development of a new ecommerce project (which may or may not be open source - we haven't decided yet). We've spent a few months (between other projects) auditing the project for security holes (found several, patched them) however OS Commerce makes very heavy use of php globals, most of it in the exact way which Zend recommends against and the very reason that Zend deprecated that feature. You will find that many web hosts will not enable php globals for you, BTW. Some will let you override the settings on a per-virtual-host basis in .htaccess but many will not allow you to use that feature at all.
Code reuse? Data abstraction? Separating data from presentation? HA! In your dreams. OS Commerce is "How Not To Code A Web App 101"
OS Commerce is the worst spaghetti code you will ever find - OpenOffice.org's code looks like it was designed by an intelligent architect in comparison.