It's not a problem with stacks, but with numbers. Lennart implies that being used by more distros means that systemd is "more standard" (my words, not his) than Upstart. Another completely fair argument would be that the number of distros is irrelevant, and instead it's the number or users what counts, and thus Ubuntu probably wins.
Because there's no mention of any option of sharing interface between both Upstart and systemd, I conclude that this feature will cause a fracture between the two contenders. And apparently both are to blame for that. It's like GNOME vs KDE all over again, only on init space.
The Linux world is full of such fractures: RPM vs deb vs anything else is just another example.