Make sure you test drive both and thoroughly inspect them. I've got a 528 and it is an excellent ride. It's not a rocket ship but it is fun to drive. Can't speak about the power of the 525. Probably ok, but now and again i wish I had the 530.
The one thing that was important to me was the model year. In 2001 BMW "loosened" up the steering to appease the soccer moms. After realizing its mistake, BMW went back to the traditional steering in 2002. Before I got my 528 I drove a 2001 5 series and did not like the steering as much as the other model years. Just something to consider. You might also get slightly better gas mileage with the smaller engine, if that is important to you; but it will probably be negligible. I currently get btwn 28-30 highway and 22 city.
You will need to decide which car is best for you after test driving, inspecting, and negotiating price.