I never really considered the Tomb Kings. What I like about the Vampire Counts is how different they are from my Lizardmen: hordes of weak rank-and-file undead rather than medium-sized blocks of highly skilled (each in their own way) Skinks and Saurus, spending my points on characters rather than elite units, a totally different aesthetic, and - of course - some bad guys.
But not just any bad guys. I like that in a world of gods and monsters, the Vampire Counts are ultimately a very human evil. They don't want to destroy the world, they just want to rule it. They - the Von Carsteins, anyway - actually want to take good care of the humans to make sure that they have a reliable source of food. An Empire ruled by the Vampires wouldn't be that bad a place - it wouldn't be great, mind you, but life would go on.
And then there's the anti-hero side. When Chaos came, the Vampires joined with the Empire to push it back. Because, I mean, they like the world. It's where they keep all their stuff. They can't let some four pushy gods come in and take over!
A lot of that could be said of the Tomb Kings as well, however. They're human(ish), they have some antihero potential, they rely on hordes backed up by heroes, and their aesthetic is unique and nifty.
I think it's the aesthetic that really did it for me. Egyptian stuff is ok, I guess, but there was something really neat about the rotten gothic grandeur of the Vampire Counts. My Lizardmen have bright colors, shining gold everywhere. My Vampires are going to be bi-tone, grim, with the dark chivraldic excellence of the heroes contrasting with the horrible gribblyiness of the hordes.