Dofollow link is counted as a backlink which is factor for increasing your site's link popularity while nofollow link is not. I would suggest using dofollow channels for your sites promotion. Regards
OP is perhaps asking whether he should make his blog do-follow or no-follow.... I would advise him to keep his blog do-follow and receive fresh comments and ideas. If he starts receiving lots of spam, then he may consider it making a no-follow... Regards, RightMan
If you want to build links from directories, you want to post in directories that are do folllow - this way you will get a share of the PR the directory has. If it is a no follow link you will not gain any link juice from the link.