What is the best hardware/GPU for deep learning?
I would recommend you read this article carefully: http://timdettmers.com/2017/04/09/which-gpu-for-deep-learning/

Basically,


The most important feature for judging deep learning performance is memory bandwidth. GTX 1080 Ti's memory bandwidth is 484 GB/s and TITAN X is 480 GB/s. Read this Quora answer to understand why memory bandwidth is the most important feature better: https://www.quora.com/Why-are-GPUs-well-suited-to-deep-learning.
Tim has also given a "theoretical" rough comparison between GPUs based on their tech specs. You can also see that in his comparison 1080 Ti and TITAN X are roughly at the same performance level.
Use 2 1080 Ti instead of a TITAN X since GTX 1080 Ti is definitely cheaper.
I've personally evaluated the performance of 1080 Ti vs. TITAN X and they are roughly the same.