I second that. Don't go with any Interspire product unless you don't care for support and continued development. Somewhere along the line the company changed from a very good company with nicely priced decent scripts, to a kind of ghost company that don't care for their customers anymore. Their support ended completely somewhere in 2010 after years of steady decline.
I still use Interspire Email Marketer because it still serves me well, although it has flaws and hasn't seen any update or support in a few years. I stopped using Interspire Shopping Cart and Article Live. They're just dead and outdated scripts.
Back to shopping software: I don't know about Interpire's Big Commerce, but I would stay clear of a company that just without warning and explanation decides to not communicate with a large customer base anymore, and suddenly can stop developing their products without any care for their users.
Back to your question: Magento is complex software if you want to do anything else than the out of the box stuff, and it has heavy hardware reqs. Like Kurtd I would suggest CS-Cart. It's a powerful program for low costs, that has been around for many years now, and is getting updated frequently. Large community as well. It's quite a safe bet. For small to midsized shops you could have a look at OpenCart, UberCart, Avactis and PrestaShop.