Warsaw or Krakow - which is easiest for travelers.
where are you coming from?
I have flown into Krakow from SF--very small airport, luggage collected within about 15 minutes of landing, nothing else remarkable about it--and out of Warsaw, which is of course a much bigger and more modern airport, but seemed to be efficiently run and I encountered no issues.
I have traveled through both. They are modern and easy to navigate if you don't speak Polish. I was very impressed with the high degree of professionalism with LOT airlines, but Norwegian and Air Berlin were also very good. If you have to choose between cities, definitely choose Krakow.
Both are fine, so fly into the one for the city that you actually want to see.
Only Warsaw gets nonstop flights from the US, and only from a few cities. Note that if you're changing planes in Warsaw (at least when I did it in 2014), you have to go through a security check IMMEDIATELY after passport control. If I had known this, I would have prepared on the plane (3-1-1 bag handy, ready to take off shoes and belt, etc).