Independent rear suspension (IRS):
1. Good on-road ride and handling.
2. Saves space...thus, allowing for more room in 3rd row seats.
3. Possibly more ground clearance down the center of your vehicle.
4. Poor wheel travel for off-roading.
5. Generally regarded as inferior in strength to solid axle in terms of towing and hauling.
6. Ground clearance changes over rocky terrain...in contrast, a solid axle always maintain the same ground clearance.
That's all i can think of now.
As for which V6 is better...i don't know. Both are excellent i am sure. All V6's of Nissan are based on the same design, which has won awards. To be totally honest, i always trust Toyota over Nissan or Honda when it comes to powertrain. You can't go wrong with either.
Just my opinion.