RM125 New chain. Which one?

I want to grab a new chain for my 79 rm125. There are so many available. Which one should I get and why? I have no idea what type does what. I know some have o-rings. But that's about it. Buying a new chain requires knowing what size sprockets I have...correct?
 
O-ring last longer but are heavy, so the inertia of rotating mass is heavy. They don't need to be oiled much because all the lube is sealed inside the o-ring.
X-ring is just better than the O because it holds the lube in better.
Non-O-ring is basic chain and needs to be lubed and taken care of. Rotating mass is light so more zip...

Are wee good?
 
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