Let me put this in simple words. If you are choosing a free blog, then I strongly suggest you to go for Blogger. The reason being that blogger is completely open source, that is, you can do anything with your blog if you have a good knowledge of programming. But WordPress free blogs prohibit its free users from accessing its core features and even prohibit them from changing the source codes. Moreover, free WordPress blogs are even not allowed to put any sort of ads even Google Adsense. They only allow users to show only their ads. But with blogger, you can put any sort of ads to it.
But if you want to buy your own domain and hosting, then I strongly recommend you to install WordPress on your host. Now, you can use any sort of ads on this WordPress blog and you can even change its source codes according to your likings. Moreover, there are several WordPress plugins which will ease out your workload.
Tip: If you want to rank your blog high in search results, then always try to write your own content. For example, you can simply research on your topic and then formulate your own ideas and create a unique piece for that topic. By writing unique content and applying some basic SEO techniques, you can easily rank your blog high in search engines.
P.S.: Both Blogspot and Wordpress have good standings and you will never be penalized for your content in terms of SEO, provided you pay your prime attention towards your content rather then blindly keyword stuffing your posts.