Aesthetics and theme is important to me. I'm taking converted Men at Arms as my First Company skellies, and converted Greatswords as my Grave Guard. My army has an Estalian theme so those big frilly sleeves and feathered hats are important for the look. My Black Knight conversions will also be sporting the feathered hat, and there's a brilliant Empire General on horseback to go perfectly with them.
Part of my consideration, however, is that my meta-game is ALL in-store. So all of my army needs to be GW/Citadel and affiliated models. I can't role up with a core of 100 or so Mantic zombies, ghouls and skeletons, even though I think the zoms and ghouls look way better. I had to restrain myself from buying the Gamezone Mournful Knights yesterday on precisely that basis. As insanely awesome as they look, its more likely than not that they would be banned on a GW store table. Plus the fluff of my army has its Blood Knights as being more Swane Knights in the vein (pun intended) of the old Lahmian bloodline, so I will likely be going with Bretonnian Knights/Chaos Steed conversions. The Gamezone Grail Knights are also insanely awesome, and since GW doesn't actually have any Grail Knight models atm, I might be able to get away with them... harummmmmm...
From a fluff point of view, your Vamp or Necromancer isn't going to be sitting there going "Mmmm... I'm most likely to encounter Skaven on my way to my goal, so I need to raise more X units to beat their Y units..." They'd just raise everything and take it along.