I don't bet but I think it'll be a toss-up, very close one way or the other, in the margin of error category. In fact, I won't be surprised if the losing side demands a re-count. The performance of Kukiz voters -- whether or not they vote for Duda -- will be crucial.
This is it. Also, many Kukiz voters may abstain from the second round altogether. How many of them will decide to stay home? We don't know. Remember, these are the so-called "anty-sytem" voters, so they may feel inclined to vote neither Duda nor Komorowski, but more of them would be on the Duda side, of course.
Also, you should take into account voters who support PO, but stayed home in the first round because they thought the ballot would bring up Komorowski as the winner of the first round, so the decisive one would be the next, final, one. The fact that Komorowski came second in the first round may cause even more of his voters turn up in the second. On the other hand, there is a large number of PO voters who feel disappointed with PO but not ready to vote PiS, so they may decide to abstain as well.
The outcome is very, very hard to predict. Opinion polls which failed completely before the first round do not seem likely to correctly predict the result of the second one.