It depends. Generally, you will get better performance and gas mileage with higher octane depending on how you drive.
If you know what your mileage is when you use 87 octane, then switch to using 89 octane for awhile and then calculate your mileage. 89 is usually 10 cents more per gallon or $2.00 per 20 gallon fillup.
$2.00 would buy you less than one gallon of 87 octane, which would take you roughly 10-12 miles.
If your gas mileage improves by 1 mpg when you switched to 89 octane, then you be going 20 miles farther on a tank of 89.
Lots of generalizations there, but that's how you can figure out if switching to 89 would save some money. Your mileage may vary. :lol:
Now that I re-read your question, the same ideas apply, except that you would probably notice a drop in mileage by switching to 87.
We use 87 in our 03 Tahoe with the 5.3L. I'm thinking we should use 89 when we tow our trailer though.