Neither. Get the 5x+ Spotify is a game changer for my run training.

They are kind of different devices for different purposes. The 5X is a lot more like the 935 in many ways. The major difference from the 735XT is in style versus function. And with style comes weight and size increase.

• If small matters get the 735XT
• If weight matters get the 735XT
  
• If style matters, get the Fenix 5X
• If battery life matters, get the Fenix 5X (or better, the 935 if you do 140.6 races)
• If a bigger, higher-res screen matters, get the 935
• If WiFi matters, get the Fenix 5X
• If barometric altimeter matters, get the 935 or Fenix 5X

The 735XT is 3 years old and potentially in the queue for a near-term replacement. So, my recommendation is that unless you have to buy a watch now, wait another 2 months to see if Garmin announces updates to the multisport watch line.