I think it really depends on how much you know about the admissions process already. The admissions talk can bust a load of myths around the admissions process, which is very useful. I personally found (with my own application 11 years ago) it much more fruitful meeting the tutor for my subject, but that's because music admissions are fairly straightforward!
Something that was nice about going to a college open day and meeting the tutor that day was that the room I met him in was the room I was interviewed in. So it was a bit more familiar than it may have otherwise been. That may not be the case with every single college/subject though (that the open day chat is the same room you'd be interviewed in)