I am curious... what would be the benefit of offline music in a watch? AFAIK you can play downloaded songs in the smartphone with Spotify while offline
The Series 3 with LTE is even better for runners than the Series 2 offers.
Many stores and vending machines accept Apple Pay if emergency food/water/medicine is required on the run. You can also make phone calls or hail an uber/lyft in case you twist your ankle or another a different minor injury while running.
The combination of offline music on the Apple Watch plus AirPods is the first thing that's improved on the experience of the clip-on iPod Shuffle and headphones. Despite some UI lag issues it's basically running nirvana.