I have always thought that the best choice is the lowest octane that is recommended by the manufacturer. In our case, 89 is recommended but 87 is acceptable. With the recent prices for gas, 89 is what I use. The electronics will retard ignition timing if necessary regardless of what octane is used. I just like to get as much power as I can so...89 it is.
When/if gas gets back up to $4.00, I'll probably switch to 87.

 

I don't race my truck, I'm pretty easy on the acceleration so 87 works for me. My 09 truck has 105k miles and probably only 10k of towing. The price difference is about 10 cents per gallon between the 87 and 89. Assuming that I've averaged 15 MPG overall, then I've used about 6300 (95000/15) gallons of of 87 grade since I bought the truck. That comes out to $630 saved. That payed for a set of tires or other things like brakes. Seems like chump change but it adds up.