It would depend on the "power" of the individual blog or forum to start with. When you mention PR5, that might be what the toolbar indicates for the home page, but there are very few individual blog posts or forum threads that have that kind of PR on their own. Remember, sites don't have PR. Pages do. And since PR is based on the strength and number of links pointing to that page, it would have to be an exceedingly popular blog post or forum thread to garner that many incoming links to start with. (Not to mention that the toolbar PR is a terrible measure of the value of a page in the first place...
And of course, all this assumes that the comment links are not
no-followed and that spiders are allowed to see the signature links on the forums (which is not always the case -- on at least two forums where I'm active, the signature links are either obfuscated or entirely invisible to spiders).
If the comment links are no-followed and the forum signature links are not indexed, then of course the answer would be that they're both equal - in that neither one of them would do a bit of good.
And in both cases, the pages in question tend to slip further down in the site hierarchy as new blog posts and forum threads arise. As they slip deeper into the archives their value tends to decrease unless they're incredibly popular articles that gain a lot of direct inbound links (which the majority of blog and forum posts do not do).
IMO, getting 10 links from 10 high-quality pages that are visited regularly and consistently by my target customers -- and thus have the potential to bring me good traffic -- would be at least as useful (and a lot less work) than trying to accumulate thousands of blog comment or forum signature links.