Actually, especially in the F/SF genre, trilogies seem to work quite nicely in either situation. The stand-alone-with-an-overall-arc series (like Jim Butcher's Dresden Files) work, but so one-long-story-arc series as well. The one difference is that with standalones, if the series takes off you can kind of get away with more than 3 books, whereas the one-story-arc books tend to need that sweet spot of 3 books.

So to answer your question...it really depends who wants to read it. ;)