I've had both. V20 is very nice, and I suppose the second screen would be useful to someone- I did not find it useful. It seems redundant to me, I could get all my apps super fast anyway, so I usually never even looked at the secondary.
Not being a Mate 9 fanboy, but you will not get better battery life than the Mate 9. Period.
The camera was nice. I want to say the Mate is better, but I think it's just easier to use. The LG did have this fancy SLR thingy you could mess around with.
All that being said, in my opinion and experience, the LGV20 was a very good second runner up, I did not mind using it, and probably still would be if there was no Mate 9.