Except Kaladin has already taken down a full Shardbearer without even using any Windrunner abilities, and has taken down another without using much Stormlight, Szeth may have a bit more experience than Kal but he hasn't exactly been practising all the time I doubt he'd outstrip him by much, plus Kaladin seems to be stronger than Szeth in raw ability, he held his stormlight for longer and most importantly they have the element of surprise, everyone knows that Szeth has unnatural Windrunner abilities, the second he bursts in everyone knows he's dangerous, Szeth would have no idea that that random darkeyed spearman is just as powerful as he is and a master of the spear to boot. My money is on Kaladin all the way on this one, especially if he any other Shardbearers are there to help out.