Illustrator is nice for designing individual vector graphic elements (such as logos) but it is not for typesetting and page layouts. It will also probably be problematic when you try to combine graphics with paragraphs of text, like typical scientific poster. This is usually, what typesetting applications like InDesign do. InDesign can be used for a single page layout as well. On the other hand, if you do not know InDesign, it will probably be overkill to learn it for creating a single poster.
For scientific guys like me, with no artistic ability whatsoever, I found using latex / beamer with some existing poster templates the easier thing to do. It also has the advantage that you can copy text and formulas directly from your poster paper. So, I would strongly advice you to use beamer, since it is also what most scientific posters are usually done in CS conferences, judging from what I have seen in demo and poster sessions.
If you do not believe me, see this which is the most frequent poster design I have seen in conferences. It takes 2 minutes to do it in Beamer but it will take many days in either Illustrator or InDesign