It depends a lot. Google's algorithms are very complex and take many factors into consideration. For example a link from within an article that has PR0 but is on a PR7 domain will have a lot more value than a link from a PR0 (or even higher) profile page on a PR7 domain. Google does know where the link is - what type of page that is (profile, forum thread, blog post, etc.) and the location of the link (sidebar, footer, author name on comment, signature on forum, etc.). How do I know Google knows? Simple, I know as well. I can build a script to detect this with a very high accuracy.
So to conclude, google does not take just one factor into consideration but the result a partial component of their system gives. For example during or after indexing a page, a component of Google's entire program might do something like this:
Code:
foreach link on page:
if link is internal:
if link is in text: link_value = 5;
if link is in sidebar: link_value = 3;
if link is external:
if link is in content body: link_value = 5;
if link is in sidebar: link_value = 3;
if link is in comments:
if comment spam detected: link_value = 0;
else: link_value = 1;
if link is in signature:
if forum spam ammount > threshold: link_value = 0;
else: link_value = 1;
And I'm not even going into link relevance, page text relevance, site relevance, domain trust, and the other tens or hundreds of factors.
Bottom line is, a context link from a PR0 page on a PR4 domain can be more valuable than a link from a PR1 profile of a PR7 domain. And that's not just because of the complexity of the algorythm but also because of latent factors like the fact that the page with the in-context link is more likely to receive backlinks from both that site and other sites, whereas the nobody has a reason to link to a profile page.
However, profile links are useful. But for them to have a measurable effect on the SERPs, you need a lot of them. As always, the key is to get your hands on every **************** link you can. Get both high quality relevant links from powerful sites and high PR pages, but also a bunch of lower quality links (if you're in a situation where you have a hard time getting better links or you can automate/outsource the process).
I hope this gives you an answer, even though it is not a simple answer.