1st of all, performance will be on par with mi3 maybe a bit worse but you get better battery life. I would personally suggest you to wait maybe like 1 - 2 more month to get a new xiaomi phone that could (COULD) use a better cpu SD660. When that time comes the cpu (SD660) will be very good and will have awesome battery like Redmi Note 4 Snapdragon.

If you feel like you need the phone now then buy it, but you will have to deal with

- mediocre camera in lowlight

- same/a bit worse performance than mi3 but GAMING is definitely better on Redmi Note 4 Snapdragon than MI3

but you get

- very good custom rom support

- very good battery life - 10-12 Hours screen on time after 1-2 days (mixed wifi and 4G/3G)

- very good thermal performance (phone does not get very warm when gaming)

- Cheap phone for the performance/battery

I just need to remind you that this is NOT A FLAGSHIP phone. The mi3 was a FLAGSHIP phone.

Don't get the Redmi Note 4 MTK because

- Horrible battery life - 5-7 hours Screen On Time after using phone for 15-20 hours

- BAD THERMAL PEROFRMANCE - Games lag after 5-10 minutes

Get the MTK for

- a bit better performance (WHEN NOT PLAYING GAMES)

- better build quality and material

I have used both and I hope my write up can help you

JUST FOR EXTRA INFO

Antutu Mi3 - 30-35k

Antutu RN4 Snapdragon - 60k

Antutu RN4 MTK 85k - 90k

Gaming is BAD on MTK

Gaming is GOOD on Snapdragon

Camera quality is about the same on both phone

CUSTOM ROM support is BETTER on Snapdragon 10/10

CUSTOM ROM support is the worst on MTK 2/10

Battery life on Snapdragon is two time better than MTK

Not cheap enough in my view.