Links from sites related to topic are best. Submission to directories next if ranked. Articles i think google values them less than before.
If you do them the same -- meaning just post on free sites, the press releases will be slightly better. If you put some effort and money into your PR, it can be very effective. Articles are only good if you post them on niche article sites where an editor actually will review it before it can be posted.
Press release sites tend to have higher reputation and higher PR. There are some articles sites that are reputable. Recently I got registered with some press release sites that required paper application to be signed and submitted. I would imagine that sites like that will get big plus from big "G". I also try to avoid paying for press releases.