Always felt it was easier to read a thread when it was 100 per page. You could miss a lot of posts by the time you got to page 18. The Jelavic Megathread being the best example.
100 posts a thread are good when using a mobile to browse the site in a shite area with slow 3g or 4g, can spend a good 5 mins reading a page instead of reading 20 posts on one page and then waiting 3 or 4 mins for the next page to load.