There are 3 factors which you should consider before buying the S9 now.

1. The difficulty is not going to come down.

2. Where do you think the BTC / BCH price will be in a year?

3. When do you think the newer machines will come out and what do you think they will cost?

Please think about these 3 factors and then make up your own mind. As of today, the earning from one S9 is 0.00135 BTC. This is from BTC Pool using a 13.5 TH/s S9 miner. Do your own math.