If you intend on hunting, get the 6". You already have a Couger that you can carry for SD. I'm getting a Ruger GP100 with a 6" as a woods carry and short range hunting gun. It will probably get night stand duty as well. I had a 4" S&W 629 but the it was too bulky and I didn't feel confident shooting it at longer ranges on deer. The 6" barrel will give you a longer sight radius adding accuracy at those longer ranges.
Also, 6 is plenty. 8's are huge and I don't like bulk. I have a 5 shot snubby and don't feel under armed. If I want more rounds, I have a speed loader as well as Biachi Speedstrips - I really like these. Again for hunting, a three shot should be plenty, so 6 is fine. Why lug the extra weight.