I played a bit of Sacrifice some time ago (not exactly when it came out, but it's been a few years still). And after what i've played, i have agreed with a review i read about the game, in a local gaming magazine. If you treat the game as an RTS then you will probably be quite frustrated with it, because of several reasons. First and foremost is the fact that the camera is stuck with you character, and so, it's hard to have an overview of your unit.
A better way to approach this, though, is to not play the game as an RTS, but rather as a sort of 3rd person Action game. Essentially, the game revolves around the main character anyway, and you can theoretically play the game using little or no creatures i think, using your other types of spells (damage, etc.). Basically it's you versus an oposing wizard, slinging spells at each other. Of course, since the AI will use creatures, it will be very hard if you don't, but that's not the point.
Now, the creatures are a bonus. Think of it this way: YOU are the big, powerful wizard with amazing powers and spell at his disposal, YOU fight the enemy, YOU crush the other wizard. And, like any self-respecting mage of huge power, you can also bring your minions with you to do your bidding.
Now, instead of seeing the game through the eyes of some (modern day) general, trying (and failing) to keep an eye on all of his troops at all time from a 3rd person view, you are a mighty fantasy sorceror, sending your minions to fight and kill and die for you, as you unleash all hell with your magic upon the field of battle and any poor bastar... erm... enemy wizard trying to stand in your way that day.
I may have gotten a bit carried away there, but the feeling the game gives you is very nice.

You (or I, at least) can't help but feel realy hyped when you're in the middle of the whole mayhem, casting spells, shouting orders at the minions (Guard me!, Circle formation!, etc. love the fact that the character actually shouts the orders at the creatures and points towards them.

), taking the souls of the dead enemy minions. Awesome game in my opinion.
Haven't played the other one though, so no idea about that one.