Which OS X Lion or ML more stable on Macbook Pro 2010 Mid 15" I had a old Macbook Pro 2010 mid 15" buy from 2nd market. And it still shipping yet on the way ...

Just know it still running the very very old 10.6.8 on it ..

Yes, I know it maybe or not have the GPU Kernel Panic problems. (I don't think all this model have this problems.)

I will try to install the latest OS X 10.10.2 on it first and testing ..

But .... if I am real the unlucky one, with the GPU KP.

And need downgrade back to old OS X.

This is I need buy Lion or ML from Mac App Store.

But ... which one more stable on this model Macbook Pro?
Your problem is more likely how you would get hold of Lion or Mountain Lion.

Unless you have previously 'bought' them from the Apple Store, they are no longer available. Apple removes old versions very soon after a new one is released.