Hey OP, So here is what I know. Publishing an abstract is much easier than publishing a paper. Publishing an abstract generally means that you have gotten a poster accepted to a conference and the conference put all of the abstracts in a journal issue. I would say (though I'm sure others know more than me 😛) that publishing an abstract is viewed on the same level as a poster presentation - because they really go hand in hand.
Publishing a paper on the other hand is a fairly big deal as an undergraduate and represents a significant amount of work. Although publishing an abstract (poster) is peer reviewed, the process is very lenient. When you publish a paper on the other hand, the peer review progress is very rigorous and often includes writing multiple drafts according to your reviewer's criticisms. Of course, being first author on a paper is a bigger deal than being a secondary author, but they are both serious accomplishments for an undergraduate student. For that matter, so is a poster presentation and abstract, just not to the same degree. 🙂