How important is to include an acknowledgment in the poster presentation?
It is acceptable to use one or both ways unless there is a restriction policy from your organization or the committee of the event, at which the poster is going to be presented. 

However, a poster is very short to present your work and ideas and you want to properly invest in the space. In my opinion, a logo is enough to acknowledge the funding agency. The assumption is that there are no other reasons to put a logo of any organization in your poster if it does not support the work in one way or another.