According to benchmarks on Geekbench, the upcoming Xiaomi Mi 6 runs faster than the latest Samsung flagship smartphones. The question of which one is the better phone won't be answered until the Mi 6 is launched on April 19.
http://www.notebookcheck.net/Xiaomi-Mi-6-scores-on-Geekbench-higher-than-those-of-the-Samsung-Galaxy-S8.213232.0.html