To contradict Highland, I have used zen-cart for a e-commerce business and it has done very well with it (ceon seo module is a must), but it does have its limitations but as its open source so anything that it did not do out of the box or there was no add-ons for, we can hire a code from eg odesk to do for you. If your on a small budget then zen-cart might be worth looking at. I also found the zen-cart community great, its not huge, but it very close nit.
I'm currently looking at Magento to migrate an e-commerce store too ( not zen-cart). Open source wise it the biggest system out there, and has a big community, lots of add-ons etc, and it does more things out of the box than zen-cart. The only issue seems to be it can be slow if the hosting is not setup right for it ( seen alot of shared hosting sites have this problem), But there are a lot of hosting companies that specialise in magneto hosting.