Relatively speaking, WM was ahead of W10M. What I mean by "relatively" is comparing them to their competition during their prime. WM was basically a little PC in the pocket.
I used WM 5 and 6 for a few years, up until sometime in 2011. I started out with PDAs, and later had several smartphones running the OS. I remember browsing the file system and even editing the registry, much like on a PC. It had a fairly robust app store available as well.
You specifically mentioned comparing both versions today, and I would give the advantage to W10M looking at it 100% objectively. WM looks very dated today, and lacks features we take for granted.