Which is better:

* Flowmaster American Thunder Muscle Truck Exhaust System
OR
* Flowmaster Force II Exhaust System - Dual Side Exit

OR are there better ones than these two

Yes!!!! MAGNAFLOW!!!!

I'm very pleased w/ the 40 series Flowmaster dual outs that I had put on. I never knew a cat-back system could sound that good.

Ihave to agree with the magnaflow. Allthough I don't have one presently, I am replacing my flowmaster with one b/c I hate the flowmaster's interior resonance.

I have a follow up question to this...There are 2 Gibson headers listed for my 99 4.8L. One is for Air Injection with EGR and the other is No Air Injection with EGR. When they say air injection, are they referring to turbo/superchargers?

No. It's emissions equipment.

Ihave to agree with the magnaflow. Allthough I don't have one presently, I am replacing my flowmaster with one b/c I hate the flowmaster's interior resonance.

exactly what I did. Had a flow master and it sounded good, but it gave off an annoying hum at normal driving rpm / around 55-65... I don't recall which version of the flowmaster it was one large muffler with dual inlets and dual outlets on my 92 K1500. Went with a magnaflow dual in-dual out (single muffler) and it been really nice. Has a nice soud to it at normal rpm, when you give it some gas it has an even better sound and yet it does not have the "hum" like the flowmaster. I like the flowmasters too, but I don't care for
the extra humming noise that drives you crazy.