You might think that you could always tell a demonology warlock on sight, its minions being bigger and more powerful, to reflect his greater dominion of demons. This isn't really the case though, every warlock's minions look the same irrespective of build. If they are over level 50 then they become easier to spot as you'll find that they'll have a felguard following them about. To be fair, he is bigger and more powerful than all the other minions.
If you're drawn to demonology so that you can summon a greater variety of demons, or maybe more exotic ones like those beastmastery hunters, then you're going to be somewhat disappointed. You'll have and use the same demons until you learn to summon the felguard then you'll have him. That said, the demonologist has an array of talents that really amplify your minion's power and also boosts your own by drawing on the power of your demon. If it is synergy with an embodiment of evil ripped from the nether that you are looking for, then this is the place for you.