Both the phones have their pros and cons, I think you should go for Galaxy C9 Pro. Galaxy C9 Pro has the following advantages over Galaxy A9 Pro.
It is not like that it is a one sided game, Galaxy A9 Pro also has the following advantages -
Verdict:- Galaxy C9 Pro has an edge over Galaxy A9 Pro but it is priced relatively higher than the Galaxy A9 Pro but considering the performance drop of Galaxy A9 Pro at a much lower price Galaxy A9 Pro also has its own charm to buy.
You can compare this on our 91mobiles link:Galaxy C9 Pro vs Galaxy A9 Pro
You can also check this video comparison
Galaxy C9 Pro vs Redmi Note 4
Though both the above devices are fantastic, but comparatively Samsung Galaxy C9 pro should be better as it has the following advantages:
Around 50% more RAM than Xiaomi Redmi Note 4. More RAM means more applications can run at the same time,
It is bigger screen than Xiaomi Redmi Note 4. Reading, browsing internet and watching movies is more pleasing experience on a bigger screen.
It is slimmer than Xiaomi Redmi Note 4. Slimmer devices are easier to carry in pocket and look more fashionable.
It is around 23% more mega pixels (MP) than Xiaomi Redmi Note 4. More resolution generally means better picture quality
It is NFC enabled, NFC enables wireless payments through your device.
But Redmi Note 4 is another fantastic device with the following highlights:
It is around 12% lighter than Samsung Galaxy C9 Pro. Light weight devices are easier to hold without tiring your arms.
It is cheaper than Samsung Galaxy C9 pro, Redmi note 4 is priced around 13K while samsung galaxy c9 pro is priced around 32K. Hence redmi has fantastic features in half of the price of Samsung Galxy C9 pro.
Verdict: So Overall if we compare Samsung Galaxy C9 pro should be better as it has better camera, massive battery, better RAM and netter all round experience. Hence If price is not a constraint then Samsung Galaxy C9 pro should be better.
You can also compare your devices on 91mobiles link: samsung galaxy C9 pro Vs Redmi note 4