I know it is too late with this kind of advice, after you developed your system, but still...:-)

I recently found nice tool WIKI for collaborative web page development, allowing for a group of people edit web-pages via browser, with page locking, version control, levels of user access, automatic creating links etc. And it IS open and free. I was impressed.

There are multiple clones, I liked one available in Perl: TWiki

I do not know functionality of your system, how much more features do you have. Still, I IMHO it might be worth to look into TWiki and learn...

To answer your question:
You do not have to open source of your system, if you expect to make living from selling source code. But, as a small firm, you may want to live by selling your services only, and you might be more credible to your customers if you can say you do not want to lock them into your system, then can improve it if they want, but do not have to - your company can do it.

Given recent economic situation: It will not be easy, but you are in full control now! Good luck!

pmas
To make errors is human. But to make million errors per second, you need a computer.