About participating a poster-session
I may not get as much feedback in a Poster Session, if I am to present it to an audience 


If anything, you may have a better chance for feedback. If you give a talk, you only have time to answer a couple of questions, whereas if you are standing beside your poster, you will probably have the opportunity to have more detailed discussions with people who stop by.


  This is just a Poster Session and that's it. Is it worthy during graduate applications etc.?


Your C.V. will presumably list your publications, and the name of the journals or conferences they were published in, but would probably not specify whether they were presented in a poster session or a talk. Of course, if you don't get to publish in the conference because you didn't attend, then you wouldn't be able to list it at all. (This does assume that there exists a full paper that is published in the proceedings; some of the other questions I've seen here suggest that not all fields do it that way.)