And the winning combination of ammo power-to-weight ratio IMO: .327 Fed Mag in either a 6 shot SP101, 7 shot GP100 or 8 shot Ruger Blackhawk--representing carry/SD, all-purpose and hunting focus respectively--looking better all the time. The GP100 would seem to come out on top in this compromise contest...though "absolute" assured durability/longevity/simplicity and reliability of function and mechanics over possibly years might favor the Blackhawk. Although I am enamored with the capacity of the Blackhawk in this sizzling new(ish) round, I will also allow that Freedom Arms also makes a .327 as a six shooter in its 97 model--speaking of reliability. All in all I keep coming back to the GP100 for all around utility, however, and just hope you're not on the island long enough to test the DA vs SA reliability issue.
Later Edit:
Until re-factoring in the 5# ammo criteria, I was all with the .357 crowd on this one--having several for the very reasons so eloquently stated in this thread. The very existence of the .327 changes the balance ever so slightly--enough for most any SD situations, and decent enough for hunting assuming for that role you're able to stalk quarry and take careful aim. You're not likely to stop a charging Kodiak dead in his tracks, but with a .357 you're not likely to either. So you've got to play the odds of likelihood here somewhere. Assuming no scope, so large (for these calibers) game will be pursued with "well placed shots" at similar close-in distances, the .327 is as likely to bring a critter down (or close enough for this exercise). If no ammo weight limitation, it'd be a .357 (or larger), and reverse this thread around and require it be a rifle, it'd be a 20" .357 carbine, hands down. But again, given the ammo weight criteria, that may just be 'cause no one's come out with a .327 yet!