How important is one's first publication in affecting prospects, assuming others are to follow?
A first publication matters less than you might think, especially as an undergraduate.  My take on likely interpretations:


If you publish as first author in a credible but low impact venue (of any type), people will think you're doing great for an undergraduate to be publishing as first author at all.
If you publish as first author in a high impact venue (of any type), people will probably assume that the senior author on the paper did the critical intellectual work in defining a high-impact research plan, and that you just executed it (whether that rightly or wrongly describes your particular situation, that is still what is likely to be assumed, given the difficulty of publishing in high impact venues).
If you publish not as first author (in any venue), people thing you're doing good work as an undergraduate on the team, but that the first author contributed more (which will probably be true).


Frankly, all of these sound like nice things to have people think about you.  My recommendation for any undergraduate is to not get hung up trying for a perfect first publication.  Your first publications will probably either involve a lot of hand-holding or else be a pretty rocky experience, as you gain experience in writing publications and communicating with your scientific community.  It's better to start writing and gaining that experience now, rather than saving it for a more painful experience with a larger and more significant work that you are more heavily invested in.