You can't go wrong with Renzo Gracie. Renzo's a great guy and I believe the caliber of the students that he produces speaks for itself.
On the other hand, I've had several lessons with Marcelo (seminars + a private). He's an excellent teacher and nice guy. He's very technical and was genuinely concerned with my understanding of the techniques. My only reservation about Marcelo is his English. He struggles to find the correct word in English at times. However, this wasn't that big a hinderance as he was still able to get his point across by demonstrating what he meant.
"Jiu Jitsu is perfect, it's humans who make errors." - Rickson Gracie
"I can take him down and at the ground I give the orders." - Ronaldo Jacare