Zombies require you to have good magic phases. If you don't, you'll suffer fairly insane losses that can't be recouped. Skeletons are much more resilient. 5+/6+ base is nothing to sneeze at. Plus you can add magic banners. Also, there isn't a reason to just do TK or VC anymore. Do an Undead list. Toss in a tomb prince into either unit and now have a WS5 unit that comes back. Also, TK magic augments are great on either unit.
If you are sticking with just VC, I would recommend skels over zombies. Zombies can be used as bunkers, or 40+ units can be used as temp tar pits. If you want something to fight alongside characters, then I would go with skeletons.