From the perspective of the SE I don't think it makes a difference. However with the actual users it does make a difference. Sig links are worse because people tend to go link blind to sigs. Hope that helps
I've found that Sig's (depending on the forum) are much better for advertising offers that relate to the forums userbase. I.e. on a Web hosting forum if you buy sig spots from users then display a web hosting offer. I'll normally link to the offer on my own site though so I can some benefit from the link popularity too
I wrote an article that partly deals with this (here) though honestly, it's a mixture of both that make forum linking a successful venture. I get clicks in my sig daily, though could likely do better if I had more that were within the webmaster niche...but when I see a thread that I can sneak a link into I find that I get many many many more clicks. However, kicking a link off in each post will likely get you kicked off a site as a spammer. Have a quality sig, post quality posts, and be a part of the community (not a bot/spammer) and when you can, post a link...but only when it's relevant. You'll find you do better.
It all depends on the forum. In many cases sig links to prove to be quite useful and some users to click on them but I think it's hard to tell. In my opinion sig links are great for advertisement purposes because they draw a lot of attention.
It's about traffic, and humans follow sig links. So even if bots don't bother, they are still effective (if used effectively).
forum signatures will effect your seo but ppc links will not effect the seo. and both will bring you visitors
Yes, unless blocked somehow. Usergroup permissions, or rel="nofollow" as two examples of sigs being blocked.
No in the Html a link is a link. Note that, many forums now have "nofollow". Usually you have to have certain number of posts, points etc to be granted having nofollow removed. So your wasting your time putting links in your sigs on those forums for SEO purposes with google, if you only plan is to post once put a link and take off..But Yahoo though will follow the links..