Anyway back to the topic. I say there's a 90% chance of Usopp vs Van Auger happening, with Usopp winning. I deduct 10% because of certain incidents in the past, whereby even though there's a sniper/long ranged fighter, Usopp did not get to fight/finish the fight with him.
The single most important thing here is that the two are direct opposites when it comes to personal beliefs. Usopp thinks that people can make themselves into something great by sheer will while Van Auger would believe that such things are predetermined and that you cannot be anything more than what Fate allows you to be. That's his Achilles heel as well: if Auger can't kill Usopp in one hit, what are the chances that he will continue to fight when he cannot determine whether or not he is fated to kill Usopp?
On the other hand, even a non-lethal shot by Usopp has a damn good chance of finishing his fight for the same reason: if Usopp is fated to be a better shot than Van Auger, then how can he afford to continue when he cannot win?
Really, the single most likely reason that the fight would last beyond Usopp hitting Van Augur even once would be Usopp talking him into resolving the situation like truly great Warriors of the Sea.
So it's not only a similarity of skills, but the fact that Van Auger is a direct counterpart for Usopp that makes a contest between the two likely.