The difference is that upstream for upstart is open to BSD/Hurd/etc. ports, whereas upstream for systemd is pretty strongly opposed to them and it does not seem they would assist in the effort.
The upstream is GPL + CLA. From a Debian perspective that's something of a non-starter. You could fork it, but then you're in the same position as you would be with systemd.
Moreover systemd has defined a bunch of stable interfaces; one could choose to expose equivalent interfaces in the HURD rather than port. Upstart seems to be in the "implementation = interface" camp.