I used to get them mixed up early on, too. They’re VERY different. And yet similar. (also I don’t think they’re ever in the same place together in the series, which is neither here nor there, but whatever) This is Santiago, we meet him in IWTV:

[memeything by ouija—bored]

Santiago is a BAMF, and he’s Armand’s right-hand man at the Theatre des Vampires, also an actor there, usually plays the narrator or character of “Death” in their gruesome stage performances. Santiago does NOT like Louis from the moment they meet. Santiago can sense weakness in Louis, and Santiago is also suspicious of Claudia just for her existence; it’s against the rules to make child vampires. Santiago is not gonna let that slide. 

Santino is a BAMF, too. We meet him in TVL, he trains Armand to become the coven master of the Children of Darkness, eventually helming the Scranton Paris branch in the cemetery of Les Innocents. More on him in TVA and B&G, which I have not memorized yet. Santino is the leader of a coven of Satanic vampires, the members of which hold a common belief that they are meant to be the scourge of humankind. I can’t say more without spoiling you, so go to Wiki for more.

You can also ask a Santino RPer! vampirexsantino would probably answer more. He’s the only active Santino I know of rn, and I don’t know any active Santiagos. vampiresantiago is inactive, but you might like perusing their archive.