If you have a paid site or blog, you really need to be serious and passionately interested about adding more content to it and making it the best site you could make it to be because you're investing money and time in this endeavor.
If you find that you're not really passionate about what you're doing, blogging can end up becoming like just another chore for you.
You would end up bored and will no longer be inclined to update your site and post more articles to it.
Being on a free site or blog, it's not that much of a loss if you decide you're sick of it and want to just abandon it.
You won't lose any money since you're on a free site.
The most you'll be losing is all your hard work, not to mention all the time and effort you used in order to write your articles and fix up your site exactly the way you like it.
But you can just move your articles elsewhere if you're so inclined.
However, if you're on a paid site, if you decide to give up halfway through, you're not just losing your hard work, you're also going to lose any money that you've invested in this.
Therefore, it's necessary to really think things through.
Consider how serious you are about your plans to blog.
If this is your passion, then go for it.