When it comes to smartphone charging, the general rule is the more watts, the faster your phone charges. Therefore 15W (the highest available form of the near-universal Qi charging standard) will power up your phone quicker.
However, your phone may not support 15W wireless charging, meaning you're better off with 10W, which your device may well still support. It's good to check your phone's specs before committing to one charger or another so you're getting the best possible speeds.