Soholaunch and osCommerce are two distinctly different "shopping cart" foundations altogether.

Both will track inventory (or you can turn that function off), and are fully featured for Customer Account management .... but are very different.
1. Either can be easily installed into your account (into a newly created Directory, of course) via Fantastico! in your Control Panel;
2. Interestingly, Soholaunch can also be used to create a website without enabling the eCommerce functions also -- it's simply a matter of choice. On the other hand, osCommerce is strictly a commerce (online shopping cart) system;
3. Seeing as how you seem to have some confusion surrounding you, it may be wiser for you to start with the simpler of the two to build your first system with, as the learning curve is not as steep and the processes are pretty much in easy to understand terminology: go Soho!

IMO

Professional eCommerce Help