Try skimming through Bitmain's s9 thread
https://bitcointalk.org/index.php?topic=1493601.0 looking at the reliability issues with them and then decide for yourself.
For the most part the Avalons run with near-zero issues. The same cannot be said about the s9's.
Plus, the Avalons are readily available from Canaan whereas Bitmain now sells using pre-order, produces in batches that sellout in a day or 2 and are not scheduled to be delivered for over a month or more.