Re: NXS 5.5x22 vs 8x32
To the point of durability, I don't think you'll ever find quantitative evidence that any of the NXS line is worse/better than any other of the NXS line. I've been wrong, but ...
As to which to get, some things to consider:
1) The 22x w/mil-* reticle is designed for ranging while at 22x. It's easy to remember and get it dialed to the proper setting for ranging that way (though some may prefer scopes that range on a lower magnification). I have it in my head (meaning, I remember it this way, but couldn't confirm via NF website) that the 8x–32x and 12x–42x scopes also range properly while at 22x, making it a bit harder to remember or get set right.
2) 22x vs 32x top end: 32x is a lot, really. If used at highest mag., it'll be harder to hold on target though recoil, amplify mirage more, pass less light, and have narrower field of view. But depending on how you'll be using it, that might all be fine.
3) 5.5x vs 8x low end: Is 8x too much for how you'll be using it?
Hope that helps.