I am in the market for a new cat-back exhaust system and have narrowed it down to one of three systems.
Installing on an '06 GMC Sierra regular cab swb 2wd, 4.8 and 5-speed.

The first is the Flowmaster Dual Outlet Rear/Side (http://www.flowmastermufflers.com/products/index.php?main_page=product_info&products_id=4081). This is the one I'm leaning towards. Always liked the FM "sound" but am a little leary of the interior resonance I have experienced with FM's in the past. This comes with the 50 series SUV muffler - sound clip here (http://www.flowmastermufflers.com/50_suv.html). Seems like a decent but not overbearing sound.
The second is the Gibson Split Rear (http://www.gibsonperformance.com/products.asp?pid=17692&make=26&model=548#548) or the Extreme Dual (http://www.gibsonperformance.com/products.asp?pid=14296&make=26&model=548#548). Best I can tell they are the same muffler but different exit location - straight out vs. angle out the sides. My preference is for the angle exit location (which I think can be achieved on the FM kit too). There is a sound clip on the first link. Sounds more raspy but maybe due to recording quality. Sounds a lot more agressive.
The third option is the Gibson Swept side Exhaust (http://www.gibsonperformance.com/products.asp?pid=14297&make=26&model=548#548). This is a single exhaust kit. Has a lot more agressive sound to it.

The Gibson website has HP/TQ rating info for the different applications which I can't seem to find on the FM site. Their claim is that they make more usable power in the 2,000-4,000 range. They also claim to not be plagued by the resonance sound. Gibson claims to get the ~same HP increase with either single or dual but a lot more TQ with the single.
I like the idea of the dual exh. that angles out the sides but is the TQ increase worth it? Or is 10 ft pounds not worth worrying about.

Any experience with either kit? Any pros or cons with one or the other?

I have the Flowmaster 40 with dual outlets on my 04 and I'm very happy with the sound! It's not to loud, but still has that nice deep sound!

I have the Flowmaster 40 with dual outlets on my 04 and I'm very happy with the sound! It's not to loud, but still has that nice deep sound!
Did you get the 40 with a kit? If so which kit was it?
How is the interior noise?
My GF had an '82 swb her dad put together for her when we met. He didn't remember which model they were but the cab noise was a little much. Sounded awsome outside though!
He also ran pipe of the headers then out in front of the wheels. I wondered if that may have made things worse or if there was no way around it.

I have a 40 series flowmaster on my 03 5.3. Its on the stock pipe and turned down in front of the axle. I am happy with it. It sounds good but its not really loud. And Iv'e only got about $65 in the whole deal.

I installed the Gibson sweptside on my 95 a few years ago. It sounds very good also. I guess you can't really see it sticking out the side.

Flows sound good along with the Magnaflow I just don't care for the Gibson sound maybe on a sports car but they just don't sound right on a truck.Just My Opinion:no:

I went for the FM system. Ordered it Monday, showed up Wednesday.
That is my project for Saturday!

on my 01 gmc 4.8 i have the borla single exhaust . It sounds really aggressive but is relatively quit inside. I would have never bought the borla but got it for 20 bucks from a friend. Now i wouldnt use anything else on this particular truck (DD).

I spent a few hours working on it on Saturday. What I read about it being harder to remove the old system than installing the the new one is true!

My initial impression is disapointment. It is quiet at idle, maybe quieter than the stock system. It has the FM sound as the idle goes up but is still pretty quiet. I was hoping for a LOT more volume.
Every other FM muffler I've been around has been loud at idle but got quieter as the RPM's went up?
My girlfriend says its way louder than before but I even had her drive around the block with me sitting in the driveway and its pretty quiet.
:(

The instal went good. No issues, everything fit good.

I give my vote to Flow Master. I have always liked them due to the extremly loud sound.

I am kicking around exhaust systems right now, also. I just don't know what to go with. We just installed the flowmaster kit on my dad's 04 GTO and experienced the same issues you are speaking of. It had the flowmaster sound but really wasn't any louder than stock.

I am honestly thinking about putting an 18" dynomax bullet on mine and having a custom 3" pipe ran to the rear corner and dumped.

Flowmaster is the way to go, I would go with dual tail pipes.:metal: