Smith is the veteran and more experienced than Koskinen. Smith has the experience and knowledge to lead the Oilers though he's struggled with injuries this season, however has shine in playoffs past. Smith is 12-9-2 with a 3.19 goals against average and .903 save percentage. If both goalies are stopping pucks at an equal rate, there is absolutely no question that Smith is a more valuable netminder because of his superior puck-handling ability.
In the big picture this season, Koskinen has been the better goalie. He's played nearly twice as many minutes as Smith. Koskinen has had a roller-coaster of a career, at times he has looked like the goalie they'd hoped he'd be when signing him from Russia but the majority of the time he's struggled. Koskinen is 25-10-4 with a 3.04 goals against average and .904 save percentage.