whats better for PR the link of your site on PR 4 directory along with about 10-15 links of other sites or a single site wide footer link on a PR 2 forum with 50,000+ SERPS in google
for every external link on the page the amount of benefit you get PR-wise diminishes PR4 would usually be better, but that may depend on the quality of the sites they are linking to.
I think the first option would be better, since google might not be amused when it sees your link on 50,000 pages of the same domain and may count is as spam. If i were you i will go for the first option.
I'm with SEODelhi Google seems to only count the first couple time a site is link to from each URL. An instant addition of 50000 links will likely just look like spam and none of them would get counted. Except.......... If the 50000 links are on a site where you might be getting regular traffic from the links, then they would definitly be a better option. You may not get any SEO benefit, but I'd would always go with actual referrals over the possible benefits of SEO