I'm usually cautious when it comes to gurus, especially if they describe themselves as such.
Ian Stewart has a solid reputation. I would also classify Jeff Sackmann as a GMAT authority, especially in quant. But as for Stephen Wolfe, I don't recall any discussions about him. That doesn't mean he's a bad choice, of course, but keep investigating before making a decision.
Is anybody familiar with Ariel Goldberg? He's apparently a GMAT tutor in Argentina. One member
praised his math problem compilation, but I don't think that there are anymore reviews of him.