The first thing to think about is the majority of fighters from each side. The Buffy/Angel universe will composed predominantly of vampires, uber-vamps, slayers, and demons of various skills. The Highlander universe is composed almost entirely of immortals. The vampires from the Buffy/Angel universe are basically dumb animals, they will kill the immortals and not think to check them for dead (See Duncan McCloud vs. Angel). Thus immortals will come back and chop off their heads. The vampires and demons are gone since they don't know how to kill an immortal. The collaterals are groups like the Initiative and Wolfram & Hart from Buffy/Angel and the Watchers and Rogue Watchers from Highlander. These guys will fight it out with no impact on the actual battle so I can ignore them. The Highlander universe only has let that ghost/demon thing that plagued Duncan in the series finale. Its illusionary power can be matched by that of the First from the Buffy universe. These ghosts will trick the other army into killing each other, which then cancels the affects. The Highlander universe is out of other kinds of fighters, while Buffy/Angel still has their most powerful members left. Even if there are more immortals and watchers than vampires and commandos there is still the bosses. I collected all the major figureheads of all the Buffy/Angel storylines into one group: The Beast (Angel), Jasmine (Angel), Kaleb (Buffy), Glory (Buffy), and Adam (Buffy). Into this group we can throw Buffy armed with that axe, since it made her stronger, and Willow since she is very powerful. The immortals are no match for this group, with the exception of Buffy since even with that axe she still is a dumbass. The immortals can't kill any of these fighters so even if it takes a while they will eventually kill every immortal and watcher (or in Jasmine's case, get them to eat them all).
Winner: Buffy/Angel Universe
Slight Correction: Not that it matters, but Ariman (the ghost like creature I mentioned from the Highlander universe) didn't appear in the finale.