How would I decide --
Two years ago, when I lived in downtown Washington, DC, I would have liked to leave my phone at home while I walked to the corner market or post office, or on a jog, while still being reachable if my wife or workplace wanted to contact me.
These days, because I live in part of suburbia with few sidewalks -- let alone anyplace that's worth walking to -- I drive all the time, and I almost always use my phone in the car via CarPlay. So, I always have my phone with me, too.
So, in the first scenario, I could get a lot of use out of an LTE Apple Watch. In the second, I'd hardly use the LTE at all.
In neither of these scenarios do I mention any need for any extra song-carrying capacity.