I've launched sites with both and here's my opinion...
Magento is like the bigger brother of Opencart. I'd say that whilst Opencart's a Lotus (nimble, adaptive, fast but awkward), Magento's more of a BMW M5 (more expensive, big, heavy but at times more practical and capable).
If you wanted to buy a ~$50 theme and adapt it to your needs then I'd choose Opencart, it will work just fine and you'll be able to dive in and dabble however you wish. It's lightweight and unless you install a load of extensions, fast on a small vps.
In contrast Magento's more set in its ways and needs significant power to get it running quickly. The default setup is superb if you want to sell more complicated products e.g. multiple varient t-shirts with a standard stamp (Colour, Fit and Size). Opencart's more awkward in this scenario. Magento will crack whatever nut you throw at it, the question is whether you need its sledgehammer.
It depends on the circumstances as to whether Opencart's any better or worse than Magento. Both are building audiences and I can see them happily coexisting. One's owned by PayPal, the other's run from a small office in Hong Kong. For them to be competing is quite an achievement really!
Not many people need all of Magento's bells and whistles and for the ones they do then a little code tweak here and there within Opencart will do it.
The other thing I've found with Magento is it's far more commercialised. Developers rarely help a customer with their modules however with Opencart I've generally found them to be fantastic.