Directory sites is not consider as link farm unless you can't moderate it very well. To make your directory site high quality, allowed only trusted website to make their profile, for example dmoz does.
If your directory sites maintained very well, then I will say don't use nofollow links also don't use redirection (Which is blocked under robots.txt, for example
domain.com/recom/some-website/ redirect to
some-website.com, and your competitor might block
/recom/ direcory in robots.txt, so no juicyrank is pass to them, but you will also not get that juicyrank, it will be wasted ).
The directory sites are ranking because of outbound links. There are so many patterns(this and this, also many more which I can't find right now) say that, actually natural outbound links help in ranking. When you use nofollow links into it, Google will simply avoid them while calculating any link metrics value, because it is made for spam prevention.
Just search on Google "thesauri reference" and you will see dmoz is ranking on top 3 position. It is because the listed website in dmoz article is actually trusted, and high quality. Here dmoz does very fantastic job, because they allowed to add only high quality website, and those website is consider as high quality because many of website also links to them. But if they allowed to add all kind of website on their directory, then Google will consider them as link farm.
So, It's all about whom you link them. If you link to good, you will actually get benefits, if you link to bad, you will get penalty. But if you don't have to time to consider which is good and which is bad, then simply use nofollow link or redirection link which is blocked under robots.txt. Both will help you to prevent link spam. And both will do same job.