Vampire and malignant units are both more powerful on their own than deathrattle, but deathrattle has more synergies going, thanks mainly to the fact that you can target them with vanhels, and they like large units of infantry with lots of attacks, which really get the most out of buffs. That, and the wight king with black axe is a tougher small game commander than the vampire lord, while providing a similar command ability, but one that only works for deathrattle units.
Both skeletons (in large units) and grave guard (in medium size units) are fantastic infantry. A bit slow, but slow, and not too tough, but lots of attacks to roll. My usual army revolves around a large block of each, buffed by a wight king commander, a necromancer, and in larger games arkhan the black, and it works alright. SCGT should work well for you with deathrattle stuff.
Also, don't let me scare you too far from black knights. I mean, the tone of disappointment about them you hear from me comes from the fact that I do run them, because I do love deathrattle as a theme, and the models are fantastic. They can be semi durable, and with such a slow army it can occasionally be useful to have something faster that you can run after an objective, but...
But yeah, that durability just evaporates in the face of anything with even the slightest bit of rend, and only one attack on those lances, not even a particularly impressive one, even on the charge, will let you down whenever you try to do anything with them offensively. And while their deathrattle keyword does let you buff them up a bit, unless you're sure they're the only unit seeing combat until your next hero phase, those buffs are almost always better spent on more numerous infantry units.
If you field them, they'll be mostly a vanity unit.
In a wound count situation, things don't change too much. Black knights become a worse deal compared to hexwraiths vs. most points systems. And while they become a better deal compared to skeletons and grave guard, it isn't by enough. Two wounds of Black Knight gets you one forgettable lance attack and 2 lousy horse attacks. Two wounds of skeletons (in a sufficiently large unit) gets you six lousy spear attacks. Two wounds of grave guard gets you 4 good sword attacks or better great weapon attacks (and which is preferable depends entirely on how much rend you see in your most common enemy units).
In a wound count context, Grave Guard are the big winners among deathrattle. Well, grave guard and tomb guard, but getting your hands on the latter is no easy feat. They're only slightly better than grave guard anyway, and even then only for the hand weapon and shield version - if you were going to use great weapons anyway then you're still better off with the grave guard.
I'm rambling. Anyway, in a wound count system grave guard and wight kings with black axes are great, and together form the core of a solid little deathrattle force that will still serve you well in scgt or really most point system contexts.
I gotta go, but I wrote some thoughts about starting deathrattle (along with other themes) over here:
LINK, which may or may not be relevant to you.