Let the user decide..
If these are my only options then my vote is for Latest post at the top.
ultimately, i thinkg the user should have a choice on how they want to view, either through User Preferences for logged in users
or something on the thread page that lets users sort it date\time wise.
What I think would be nice is..
(in this case oldest post first seems better)
1- Let the first post be the original post
2- have all the posts on the same page. so that i dont have to navigate between pages. This also would help me use the browsers search box to go to a desired post faster...instead of trying to remember on what page i saw a particular post, navigating there and then finding it. This obviously makes sense for posts that would span pages.
3- categorize the posts into groups of (total posts for the topic)\(some meaningful number)..e.g if 100 posts then say groups of 20. All the posts need not be visible when the user first views that page. Show post 1, followed by hyperlinks to expand to posts 2-20, 21-40, 41-60, 61-80, followed by the latest 20 posts
visible 81-100.
4. When the user clicks on the hyperlink, it exands those 20 posts on the same page, maintaining the same sort order.