Which init system for Debian?

Hmm? Systemd is pretty good about documenting their interfaces[1] and keeping backwards compatibility[2]. If you stray from the defined interfaces, then yeah, you might find some landmines. Which APIs are you referring to being in flux? Anything related to D-Bus[3]?

[1]http://www.freedesktop.org/wiki/Software/systemd/Interfac...
[2]http://upstream-tracker.org/versions/systemd.html
[3]http://www.freedesktop.org/wiki/Software/systemd/dbus/