For what it is worth (purely anecdotal, here):
I am running one ASIP server (files, web) and two OSXS (files, web, email, POP, majordomo). The ASIP and one of the OSXS machines are old (PowerTower Pro 200 and PowerMac 7500/200 respectively).
Both are very responsive (but the load is light). The ASIP has suffered significant stability problems. Not so the OSXS. I prefer the OSXS, particularly since I would expect my investment in that platform to carry forward to whatever server platform Apple will provide in the future.