First off, just about any well known name brand is probably better than the stock one. Even the low end model.
1. The non-o ring models are probably both the same size as the stocker. Both are probably way better in quality. The x/o-ring models are wider to fit the rings in there. DID is a very good chain. Good materials and tight tolerances.
2. DID is better from EK from what I've heard but I'm sure both are way better than the stocker. Don't know about IWIS.
3. This is where I spend all the money I can. Money largely does buy quality here and changing the chain and sprockets is a pain, better to do it less often. I change them all together. Again, maybe overkill but gives me the longest life from all parts. X-rings seal better, last longer, and have less friction than o-rings. X/o-ring chains last longer mostly because try as we may, most of us suck at cleaning and lubing the chain. Also, they don't get water in the bearing surfaces when it rains. On the flip side,when one link goes bad the chain is done, no way to get new lube in there. I would go for the high end x-ring chain. It's overkill but will last longest in reality. I put it on my last bike and will use it on the C5 when I'm done with the original.
BUT it may not fit your bike. It seems the G5 may not have enough clearance to fit the
wider girth of an o-ring chain. If you can pick one up locally try to link it to your old chain with the master and pull it through. If it doesn't fit get a good quality non x/o-ring chain and clean and lube it well.
Big singles and twins are hard on chains. I guess it's the fewer big hits of the piston compared to the more frequest but smaller on multi cyclinder bikes. Pay for a good chain.
Scott