Don't underestimate the time required to develop a successful blog. Writing doesn't come easily to most people, and you have to post regularly.
You may load more time on the front end setting up an ecommerce site, but as you get it going, the site itself will probably require less time to maintain than it did to get set up.
How much time overall it will require to run an ecommerce site will depend on what you're selling. If you're selling tangible goods you have to pack and ship, this will require more time than if you're selling electronic goods (ebooks, downloadable software, etc.) or if you're a drop shipper who doesn't stock your own merchandise.
While a blog can be set up fairly quickly, it requires a good chunk of ongoing commitment (if you want it to be a high-quality blog, that is). And if you want it to generate a consistent stream of decent income, it needs to be a very high-quality blog. It will take a long time and a lot of work to build traffic and reputation to the point that you can sell ads for any kind of money.
In short, neither one is a shortcut to easy Internet riches.