I think that already Marc is a better defender then Pau and more aggressive, but at the same time I don't ever think Marc will ever be the skilled big man Pau is. Pau is one of the best passing big men in the league, and very skilled around the basket. Marc has great work ethic, a big body to bully players around, and a pretty good offensive skill set, but not as good as Pau. So will Marc ever be as good as his older brother? Discuss.
I think their games are different. Marc is more of a physical, powerful center while Pau is a finesse PF. I don't think Marc will be as good as Pau but he definitely will be a solid starting center in the league. Pau is more versatile IMO, but Marc will be a much much better defender, and probably already has the edge on defense as you pointed out.
kobimel got it right. Their games are pretty different, but I don't see Marc beeing in the same level as Pau. It's hard to tell right now, Marc is still young at 23 years old.
Marc is a great defender for his age. He has the same problem as many European rookie big man which is staying out of foul trouble. Marc is bigger then a lot of the seven footers in the league, over 180 lbs, and he uses it to his advantage when in the post. I think he has a bright future ahead, but not sure if he will reach Pau's level.