I would suggest to have Premium membership and it seems worth as per prices and the advantages offered. Do follow active signature links and visible for not registered members as well, so good for SEO.
Nofollow prevents search engines (like Google) to index your site content. Dofollow allows search engines (like Google) to index your site content, but there's downside - your blog with comments and links with also get indexed. You can check this blog post.
I personally think in terms of signature links it doesn't make a difference if there'r do-follow or no-follow. Forum links are bad for SEO.
No follow dosn't matter for popular sites. It's simple example is FACEBOOK. Facebook is nofollw but every SEO want to share his post there.