I have no guess about what Congress will do in this instance (but see Sarah Binder for an excellent analysis of how these sorts of votes are generally handled). But I am confident that a defeat for Obama’s plans will not set any sort of terrible precedent. All it will tell us is some combination of the president having a fairly weak case at this point, or the president having done a bad job of selling his case. To the extent that the former is true, the nation — and in the long run, Obama — are better off if Congress opposes him. To the extent that the latter is true, it will only mean that future presidents have a stronger incentive to sell their policies more broadly, which sure seems like a good consequence to me.
There are all sorts of legitimate reasons for members of Congress to vote for or against authorizing military force in Syria. But absolutely no reason they should worry about the precedent it sets if they vote down the president.