May I sugggest that for Free vs Enterprise, you consider making them into one compilation, but make whatever "bells and whistles additions" you want for an Enterprise version, be modular and pay-for. Or accessible for download only by sponsors, or something along those lines. I know that more clients I recommend NopCommerce to would be more then willing to pay for certain things, if they need them. I think overall, the more modular you make NopC for 3rd party developers, so that people can release either free or pay-for modules (gateways, etcetera), the easier the whole design will be for all involved, and should garnish more income for the core team. That is just my 2 cents worth. At the current exchange rate, I may owe. :)
I hope you catch my meaning.
Summary:
Make it one release, and make certain portions that you would consider Enterprise (pay-for). In this way, you would be supporting one Free version with source of those free components. And then you could release modular libraries that would add additional functionality for-pay. Anything you can do to help allow for turning modules on and off, would be brilliant. Above all though, keep it simple for developers (internal and external) to add to the system and turn things on and off.