I have owned 4 different SSR's.
Two automatics and two 6 speeds but all with the 6.0 Liter.
I have driven the 5.3 Liter but don't plan on owning one. I am at the top of the SSR wish list.
There is about a 100 HP difference between the two engines and the SSR weight is just North of 4,700 Lbs.
Both engines will make the SSR get up and go. The 6.0 is just a little bit faster.
If you want REAL fast then it is going to cost you.
Contrary to what you might read on the forum the 6.0 runs just fine on regular.
The owners manual only states that you may notice a reduction in performance.
I use the middle grade gasoline (89 Octane) now however I used regular in the first two SSR's I owned without any prooblem.
The 6 speed gets better mileage. I would get 22 mpg on the highway.
Loud exhaust lowers the mileage on all SSR's.
It has something to do with the sound and your right foot.
Good luck with your search.