either way
The most important thing when you start out to think about scalability! Of course, at the beginning it is really important to get as many thing for free as possible - so you loose much less money if you fail, also you can earn more money later if you get traffic.
So the question is:
What would you do if your blog is succesful? If you have many visitor? Most probably you would want to buy bigger hosting plan, or move your site somewhere, and that is the point where some user fails:
You need your own top level domain (and own hosted script) from the beginning. What would you do if you want to make some changes in your script in blogspot? You can't! If you move to new (own hosted) script (like joomla, drupal) you loose visitors!
Also what if you buy your own domain later and move your host? You loose visitors from search engines, the same case if you change script - although you can redirect your traffic but on a few hundred or thousand indexed content it can be a nightmare!
So at the beginning, i think you should:
- buy a domain
- use own hosted script (like wordpress) on free hosting (there are few decent ones without much downtime - maybe not top hardware, but come on, with legal content you need time to build traffic)