I got the LS2, so I'm pretty much stuck with using 91+.
As far as the 5.3 and whether it would run better on 87 or 89 or 91+, I have to say that it could depend on many factors - weather, altitude, quality of the fuel, any engine or programming modifications, etc. I do know that for the past month+ I have been using 89 in my still stock Silverado and noticed that after the second tank that power had improved and mileage is up by about 1 mpg. Is that enough to warrant it? It's up to you to decide. I just got my westers computer yesterday, so we'll see just what the 89/93 dual tune can do.
There is not much difference between Regular and Premium in my 2004 5.3L. Sometimes the fuel economy and power seem bit better with Premium, but that difference is at best minor and could be due to other factors than the Octane rating.
It would be interesting to know what the engine calibration is relative to detonation threshhold. If GM set the basic ignition calibration above that level and then uses the knock sensor to back out spark advance, there might be an improvement in power and economy.
As far as ethanol content in the fuel, it's only advantage is political (farm subsidies, tax credits, etc). Ethanol has about half the energy content of gasoline and and adding 10% to the mixture drops the fuel economy by 5%. If you fill up with an ethanol blend, make sure it is priced cheaper than 100% gasoline; it is worth less in driving range. (Worse yet, ethanol gives back less energy as fuel than went into growing & harvesting corn and processing it into ethanol according to a US DOE study published in 2001).
Had an 03, put 11600 miles with 87 octane, now have 04 put 7200 with 87 octane no problems with ping or low power/ looking for 05 just to get the 90 HP/ will never be without an SSR for the rest of my life/ May give up my 01 Prowler but never my SSR without already having another SSR in the drive.