What is recommended: a) Posting (which has link to your website) in old pages of a forum or blog (which has high PR value) b) Posting (which has link to your website) in new pages of a forum or blog (which has low/no PR value) Also, which is better: a) many links from 1 website to your site. b) 1 link from many website to your site.
In my opinion: 1. A Higher PR pages helps u to increase your PR (only dofollow links) Commenting on new posts will give you PR0 backlinks, but remember this is a permanent link where the page might gain some PR in next update too. 2. B A combination of .com .org .net .gov .edu will be a great mix.
I would recommend you b) Posting (which has link to your website) in new pages of a forum or blog (which has low/no PR value) and b) 1 link from many website to your site. Reason for 1 - PR is outdated parameter.Posting comment on a new page will benefit your site in both ways, in terms of traffic and search engine. Reason for 2 - If you get back link from different sites it will look natural and add more value then links from single site.