Axe + focus has great synergy but, as much as I love that weapon combo, there's too many 'ifs' happening to make it all happen. You have to actually land the focus hits and stack the vuln on a moving, dodging target - bare in mind these can be cleansed etc even when they do hit. It also offers almost no defence compared to the staff and the offensive payoff just isn't really there.
Staff is a lot less conditional as the marks can be triggered by pets etc to cleanse yourself and apply aoe dmg into stealth, plus it gives the aoe fear to interrupt stealthstomp and kite/chase from range. I'd pretty much settled that it'd be staff, it's really the trusty backbone of any necro build in pvp.