I think you can list both, if you'd like. Gutonc is "a real researcher". Has a PhD and all that jazz. In his/her world, correctly, a poster presentation is noise and not worth listing.
In my world (and probably the OP's world), the poster may seem like a "bigger deal", and I think it's fine to list both. Shows that you did a project, presented as a poster, and then followed through and got it published.
If you presented the same project/poster at multiple meetings, I would choose only one to list, whichever you think is the best.
Bottom line is that it's not going to matter much either way. I agree that the manuscript will hold much more weight for those programs that care about these sorts of things. It would be interesting if ERAS could change their system to better address this -- have you list a project, and then all outcomes (i.e. posters, manuscripts) from that project.
If you (or someone else reading this in the future) has lots of manuscripts, then I wouldn't clutter your application up with poster noise at all. But if you only have one project with a poster and a paper, it's fine to list both.