As stock, there is not much difference in the actual HP between the two. I have heard from some good sources that the 525s are pushing out between 535 and 545 in some cases (there is a thread on this board as well). This was a dilemma when I bought my boat and based on all the people I spoke to, they said the 525s were the way to go because stock performance in my boat was only increased by two maybe 3 MPH., for a substantial increase in cost. My best friend has a 02 29 Fever with the 575 and there is basically no performance difference except the 2 MPH top end. Now he has messed with the boost a little and ECU and is seeing 86MPH. Quite higher than my 78-79. I guess it would boil down to how much they are charging for the 575s. Since Merc is discontinuing the 575s it might be a good time to buy.
I love the 525... and he loves his 575.