Open source (free) shopping cart and closed source (paid) shopping cart. Some people think open source shopping cart is better, someone not. Which one is better? What do you think?

What ever works for you smoothly is best for you.

First of all find software with interface and functionality suit your needs. See/find as much variants as possible ( Hotscripts.com ).
Closed source software becomes "open source" due to hackers in 2-6 months. Guaranteed.
Before you buy any software - go to Google and search for software's title in quotes: "Shopping Cart Name". If you'll find pages with titles like "Shopping Cart Name Nulled" (you'll find them for sure if script is more than 6 months old) - check if these pages contain download links to the software: if they all broken (removed) - developer really care about security and it's a good sign.

In dealing with others money always prefer a software which gives you support never prefer a software which is not support professionally