Hmm.. These are all really looking like good options
I've never tried a three point sling, mostly because i don't have my own gun to try it with, but i think that might help a lot, just to get a lot of the weight off my arms, because i am mostly lacking upper body strength (which i have been working to improve). I don't think wearing the vest and carrying mags/water/etc will be too much of a problem, because i have done hiking and stuff where it was necessary to carry heavy backpacks, and i've never had a problem.
So far i'm really liking the idea of the P90.. I've been reading reviews of it, looking at pictures, and checking out specs, and it seems like it would work very well for me.