Depends on your goals. If you want simple and are just starting off and testing things out to see if what you're offering will sell, then Shopify.<p>The biggest problem I see with Shopify is customizing the checkout. The checkout process is hugely important in optimizing the site for conversions, and to lose control over that piece of the site is a difficult trade off.<p>I used Volusion for my store, but they've gone downhill. I would probably end up with BigCommerce these days.<p>Magento is the best out there, but only if you've got the money and time to spend. I wouldn't walk into a Magento store unless I had $50k to spend and was very sure I'd bring in revenue of at least $500k a year.<p>If you're not there yet, go with a hosted cart so you can focus on the marketing and get to market quickly.
For relatively simple stores, Shopify's the best. I have yet to meet someone that can't manage their own store through it after I set it up for them, and the app store provides incredible value. The stuff you can plug into your store is so much more polished and higher value than the "plugins" available for most self-hosted carts. Want to run a remarketing campaign to bring back window shoppers? There's an app for that. Want to e-mail people who abandoned shopping carts with a coupon to finish their purchase? There's an app for that. Want an e-mail telling you the traffic source of each sale when it occurs? There's an app for that. Want to plug in your favorite analytics companies without touching the theme code? There's an app for that.
If you're familiar with advanced PHP, Magento is a great choice however be prepared for a lot of head-banging. I'm pretty comfortable with Magento and it's one of the more advanced self-managed eCommerce platforms out there that handles everything from stock control to integration with every payment gateway imaginable. If automatic is more your thing Shopify is a great choice.
I've been using WooCommerce on WordPress recently to build an eCommerce shop, and keep butting heads with it. It's incredibly hard to theme and tweak. Plus, there's so much WordPress stuff that is redundant in it. It's rather much a mess, and I wouldn't overly recommend it, even though I really wanted to love it.
See FastSpring if you're looking to sell SaaS or downloadable software or games. No upfront or monthly costs, limitless, rapid-response support, free setup, and it's uniquely full service.