They are two different things. The Visa Meet N Greet is in Innovations West. The Epcot character spot is just across the walkway in a different building - you walk just past the walkway that heads to The Land and Nemo and you'll see it on your right.
We have done the Visa Meet N Greet numerous times. The characters have always varied, but Mickey has always been there. Once it was just Mickey, but the last few times it has always been at least 3 characters. You can have your picture taken with all the characters that are in the room together at the same time. You can get a free 5X7 of one of your photopass pics taken here.
At the Epcot Character Spot all the the fab 5 are there, but in separate spots, so you wait in line, have your pic with Mickey, then move on to Minney and so on.
We love both - the character spot is nice because you are guaranteed a pic of each. The Visa thing is great because you never know who you're going to see AND it's a private thing - just your group is let in and the characters really play with the kids (and adults!). Worth the wait for us every time.