I have driven both...a Laredo 3.6 and a Limited 5.7. To give you my frame of reference, my current daily driver is a 2002 GMC Envoy with the 270hp 4.2L I6 but I regularly drive my Mom's 2007 Cadillac Escalade with the 403hp 6.2L V8 and my fun car is a Corvette so I'm used to power
I drove the V6 first and was underwhelmed overall. I wouldn't say the vehicle was "slow," however from seat of the pants it definitely felt slower, or at least less responsive, than my Envoy which I thought was disappointing. I don't consider my Envoy fast by any means, however it does have a decent punch when you push it and it kicks down a gear or two. I think the JGC also "felt" slower due to the fact that it was so quiet and you don't really hear the engine much...when I looked down at the speedo I was going faster than I thought I was. But on a couple of dead-stop acceleration runs I was left wanting more. To top it off this was a total base model with no options (NONE) so while the interior felt well put together and quality it didn't "wow" me.
A couple weeks later I sat in an Overland and then drove a V8 Limited. First of all the upper trims totally transformed the feel of the vehicle...the two tone dark/light frost Overland left me drooling. When I drove the V8 Limited I came away with a smile on my face
It felt comparable to the Escalade when I pushed it (which is a good thing) and never felt underpowered. It did however seem pretty docile off the line and when cruising around unless you pushed it...I guess that is a matter of gearing and also goes back to how QUIET it is inside. I drove in Auto the whole time though and didn't think to try the Sport mode. I did notice a very loud, annoying ticking sound from the V8 in the Overland (which I just cranked up but didn't drive)...I was walking around the vehicle while it was idling. I didn't notice this at all on the V8 Limited and didn't think to listen for it.
Personally I came away sold on the V8. I REALLY wanted to like the V6 as I am not thrilled with the fuel economy of the V8 and was hoping to replace the Envoy with something a little better on gas (I get about 15-16 city/20-21 hwy with the Envoy) and the Escalade is downright painful (the way I drive it is 13 city/16-17 hwy on premium...but this was the first model year before they implemented the cylinder deactivation technology in 2008). Plus the upfront cost savings of the V6 was attractive
But if/when I place an order for an Overland (waiting on the Saddle interior and technology package) it will definitely have a V8....YMMV. (I also occasionally tow so that factors in the equation as well, but the Envoy has been sufficient for my needs for the most part so that wouldn't make or break the V6)