The latter is currently not up to date in portage nor integrated into our new baselayout, while wpa_supplicant is both up to date and well integrated in basesystem (thanks to Uberlord). We currently have a few open bugs for xsupplicant in our bugzilla, including a a version bump (bug #100680) and the design and development of a new rcscript for integrating it with our new baselayout (bug #98253). Also, wpa_supplicant seems to be much better supported from upstream than xsupplicant; more frequent releases and much more wide spread usage.
To justify having to maintain two packages (along with rcscripts) with the exact same purpose, I’d like to hear from xsupplicant users why they prefer xsupplicant over wpa_supplicant – and what prevents them from moving to using wpa_supplicant instead. Please post a comment, write me an email – or contact me on IRC.