Which logo to use in a seminar title page?
The acceptable use of logos is usually described in an institution's corporate design manual. The corporate design manuals I've seen do usually not allow people not affiliated with the institution to use the logo without explicit permission, so I would never use the host institutions logo. Such thing will also not vary between fields (in my university's cd-manual, only some color schemes differ between faculties). There might even be some legal issues involved, but I'm not competent to discuss legal matters. 

Most academics in at least my field, quite reasonably, ignore corporate design manuals which might even specify minutiae of how your slides are supposed to look. Usually, they do this by not showing any logo at all- my preferred choice for aesthetic reasons. But there is absolutely no reason to show a host university's logo.