Which one is harder to beat?

Berlin. I don't play either though, but that's my opinion, there are actually many sharp lines in the petroff despite it's drawish reputation.

For who?

I seem to do really well at my club with the Petroff, but in OLC, I tend to get mowed down.

Given that it says "Which one is harder to beat?", and we are talking Defenses, not Attacks, that would imply that they are asking which is harder for White to beat.

I would have to say the Berlin is probably harder to beat than the Petroff, but the Berlin also probably has a lower win ratio for Black, though I have won with it many times.

Which one has a lower win percentage for white?

In which database?

My point was that some people will find the Petroff harder to crack and others will find the Berlin harder to crack... 

IMO, play whichever one you prefer to play!