The two parties are not very different. Remember they were in a ruling coalition once, and a few people moved from PO to PiS after PiS came to power.
Poland has no true left wing party anymore, and there are more extreme right voters too. Something is stirring on the right. PiS and PO are different shades of centrist I think. Especially that what PiS does and what it says are two completely different things.
Funny that you say that, because in my opinion Poland lacks the true right wing party. All major parties are left wing, with their economical programs being slight variations of socialists wealth redistribution, and government regulation of pretty much every domain of life. PiS might pretend to be patriotic and conservative but they are following Marx teaching more than God ones in many areas.
Poland has no true left wing party anymore, and there are more extreme right voters too. Something is stirring on the right. PiS and PO are different shades of centrist I think. Especially that what PiS does and what it says are two completely different things.