nofollow is a HTML attribute value used to instruct some search engines that a hyperlink should not influence the link target's ranking in the search engine's index. It is intended to reduce the effectiveness of certain types of search engine spam, thereby improving the quality of search engine results and preventing spamdexing from occurring in the first place.
The nofollow attribute value is not meant for blocking access to content or preventing content to be indexed by search engines. The proper methods for blocking search engine spiders to access content on a website or for preventing them to include the content of a page in their index are the Robots Exclusion Standard (robots.txt) for blocking access and on page Meta Elements that are designed to specify on an individual page level, what search engine spider should or should not do with the content of the crawled page.
dofollow is a HTML attribute value used to instruct some search engines that a hyperlink should influence the link target's ranking in the search engine's index. It is intended to increase the effectiveness of certain types of search engine.
Ater those description of the "NOFollow" and "DOFollow", we can said that the DOFollow site actually provide a linking to your site but the "NOollow" not.
If you are looking for improving the PR of your site by commenting to other site (whether blog or website), then you have to go for DOFollow Blog/Website.
On the other hand, to attract your visitor to comment on your website or blog, make your site to "DOFollow" is more effective. However, we shall be careful about the spam on your website or blog if you are setting your site to "DOFollow".
Here provide one blog with PR4 which is "DOFollow" : http://freetipsforyou.blogspot.com/