Apple has reserved 8GB of the 32GB for music in the current WatchOS. Similar partitioning seems to at one point or another have been enforced for earlier models-- trying to get an authoritative answer on this point is a little murky online. As I understand it, this was to prevent there not being enough room in which updates can be performed. In any case the non-music storage is available for photos, podcasts, apps, etc.
Maybe I'm the only frequent international traveler around here, but one thing that swung me to the 5 was the international emergency alerts and fall detection. Nice peace of mind there. I keep bringing this up but no one else seems to regard it as desirable!
Also, based on my few weeks with the 5, I gotta say: AOD is one of those things you miss when you don't have it. (You can quickly turn it off by engaging Theater Mode. I did that on a long flight yesterday, turning it back on when I landed 11 hours later. By the time I face-planted in my hotel, the Watch still had more than 40% battery 28 hours after first putting it on.)
Anyway, both are great watches and I'm really happy with my 5. I do wish the podcast-sync situation would see a resolution soon.