I would use 87 and save your money. Higher octane is not about giving you power, its about preventing predetonation, etc. in high compression engines.
My SVT Focus calls for 91+ and I use 93 as that is what is readily available in my area(not 91).
When I had my Contour I used 87. Seems like the couple times I did use 89 I got worse mileage than on 87.
I would just use what it is the manufacturer recommends.
http://auto.howstuffworks.com/question90.htm
