There is a HUGE difference between a caster and a commentator.
Caster is a person who can do the rapid fire play by play and paint a vivid picture of what is going on while simultaneously hyping the game.
Commentator is a person who talks ABOUT the game, i.e. why people are doing what they're doing, why it ssignificant, what they are going to be doing next, general matchup stuff and strategy etc.
Look at Tasteless and Artosis from the Starcraft 2 scene. Look at Husky and HDStarcraft from Starcraft 2. In both of these cases, the former is the caster who is an awesome, charismatic person to listen to, while the latter is the "analyst" commentator who talks about technical stuff. Obviously there is overlap and synergy between the two, i.e. the caster will set up the commentator for something to talk about by maybe asking a question or using allusions.
If I had it my way I'd probably have a duo along the lines of
9.95 and
Pig Of The Hut , if you've read my post at all you'll know who fits which role and why. I haven't heard enough of Arturo's commentary to really comment one way or another. I know that he is going to be one of the guys there and props to him for his position within MLG, but I don't know where he really sits on this spectrum of caster vs. commentator. Maybe he's gonna pull a Day[9] that is pure man-mode and can do both simultaneously and flawlessly. Time will tell.
TLDR there is a difference between casters and commentators and I hope to see one of each for MK9. Chemistry is the most important thing of all, though.