Personally :
- I don’t new the fancy GUI provided by Proxmox (and I’m not a huge fan of it, I much prefer Xen Orchestra from XCP-ng project)
- Excellent and consistent CLI experience with the provided client (local or remote). With PVE, sometimes you have to use the CLI, sometimes the GUI, I don’t know, it feels less consistent than LXD.
- Cloud like features such as storage volumes, projects, profiles, image store
- Much more possibilities with Cloud-Init through profiles (Proxmox CI integration is very limited)
- Container experience is much more polished and can provide latest improvements in kernel area (shiftfs, id mapped mounts, pidfd)
Both of these OSS projects are really good, I think it’s a matter of preferences. I like a lot the “minimalist” aspect of LXD. Previously, I was a Libvirt user (with virsh and so on), so that’s why I prefer the minimalist approch, and with the recent support of virtual machines, I’m much more happy with LXD (and you don’t need to deal with miles of XML structures to configure your machines 
).