looks like I had a harder time than most other people here...
when I first developed my army concept, cursed immortal Kislevites fighting an eternal battle against the forces of Chaos, I compared the two undead armies with the sources on Kislevite forces (and also some real-world Russian/Polish/Lithuanian historical and folkloric stuff)
Tomb Kings offered undead light cavalry (for ungol horse archers), medium cavalry (winged lancers), chariots (war sledges), and ushabti could make passable undead bear-monsters
On the other hand, Vampire Counts offered Varghulfs (a much better undead bear-monster), wolves, heavy cavalry (gryphonne legion), corpse-carts (war sledges), the black coach (maybe a war wagon), spirit hosts (ghosts of Kislevites past) better characters, and more flexible magic
Unfortunately neither army offered core infantry with great weapons (both have special infantry with great weapons, which is something at least). Also, both armies offered plenty of options that didn't fit my image at all... tomb scorpions, casket of souls, bone giant, skull catapults, or zombies, ghouls, bat swarms, dire bats.
I ended up going with VC because, at the end of the day, the fluff is easier to justify. Even if TK were the most perfect fit rules-wise for what I'm trying to do, I'd probably get sick of explaining to people why I modeled my Nehekharans as Kislevites, whereas vampires can show up pretty much wherever in the Warhammer world they want.