In choosing blog directories to submit to, there is the choice between the free and paid directory submission. Free listing means that there is no charge for the review or listing of the site. Paid submission will require a one-time or recurring fee for reviewing or listing the submitted link to a blog directory.

What is the Similarity?
Blog directories, whether free or paid, list blogs according to category. This therefore makes it easier for human searchers to find what they want as opposed to listing by keywords usually done by search engines. Categorization would depend on the contents of the blog in general.
Depending on the directory, submission can be on automated acceptance or evaluated manually by editors. While some blog directories would accept all and for free, some are pretty selective and require a fee. It has to be made clear though that acceptance of all and free as well as being selective and having a fee does not always come together. It is possible that free blog directories can be very selective. The acceptance of blogs would usually depend on the objective of the blog directory. It is possible that both free and paid directories can be selective specifically for those who are intent on giving their users the best search experience.
What is the Difference?
Aside from the payment aspect , there are differences between free and paid. As expected, blog owners are paying for something and these would come in the form of services and privileges. They may come in the form of allowing the input of several keywords or keyword phrases. This is very important to ensure that blogs are also more easily found by search engines aside from humans.
While it is common for blog directories to accept the submission of home pages, paid blog directories can also offer submission of inner pages as well. Paid blog directories are also typically maintained by a dedicated staff who will make sure that they are clean and spam-free. In the process, they tend to develop authority which is always good for the blogs included in their listing. It is also highly likely that loyal visitors of blog directories will form a target traffic to listed blogs. There is a thinking that the quality of visitors of free and paid blog directories may be different.
Making the Choice
Paid directory submission can provide certain advantages but if you find that it is not yet possible at this time, go for quality blog directories, whether free or paid.