Personally, I feel I take away a lot more from x isolated talk than from x talks at one conference day.
For the seminar, especially if it is a sequence of talks from one book or article where one "works through" something together I would especially recommend several meetings.
In the best case, students will try to actively digest the talks afterwards and/or prepare for the next one.
It is then also easier to address questions that might come up yet cannot be answered on the spot, by reporting them to the beinning of the next session.
Now, in the worst case, students do not care at all and forget everything from one meeting to the next. But, first, this is pessimistic a view and second one can work against this by briefly coming back to last weeks talk at the beginning of the session.
However, there are also considerations against weekly meetings:
Sometimes attendance can be a practical issue. Either since students simply do not show-up sometimes if it is weekly, or since it might be difficult for them to do so (in which case one might try to help them voa a different form of organizing it).
If the presentations are by and large independent there might be reasons for fewer longer session, perhaps even a full day, and to seomhow give it a mini-coferrence feeling. Yet, also here, there are practical considerations, such as: Is finding a room for a full day perhaps a problem? Can students be available for a full day?
In brief, in some cases like "working through" a book I would strongly favor weekly meetings; in the other cases, I'd say it depends mainly on local, practical circumstances.