I use 93. But only because the station I use only has 87, 89, and 93. Otherwise I'd use 91.
The manual says "The 3.5L and 5.7L engine (with automatic
transmission) is designed to meet all emissions
regulations and provide satisfactory
fuel economy and performance when using
high-quality unleaded gasoline having
an octane range of 87 to 89."
"The 5.7L engine (with manual transmission)
is designed to meet all emissions
regulations and provide excellent fuel
economy and performance when using
high-quality premium unleaded gasoline
with an octane rating of 91 or higher."
I added the underlining and bold text.
The way I see it, IMO, it's acceptable to use 89 (with the auto trans) but 91 is desirable.