I think that it would be better if it was Kagami versus Obito based on the following:
We all know that the previous generations of ninjas were much stronger than the current ones, prime example Hirashima, Madara, Hiruzen, Tobirama, and some of the first-second-third gen Kages from the Stone, Cloud, and look at Hanzo (took on the three Sunin, even named them and had the upper hand but decided to let them live now that's some power).
I say that to say that it is possible and would make more sense plus better story that Kagami is Tobi. Kagami trained and fought along side the best of the best and might even have met the real Madara as a child. So Kagami could have been really powerful he was on the same team of the Anbu Black Ops that Tobirama "2nd Hokage" lead and Hiruzen "3rd Hokage" was on (oh yeah and Danzo).
And some of you have already mentioned that Obito dying at that time as a weak and young ninja only to come back a year or two later to take on the 4th Hokage???? That doesn't even make sense, you dont get those types of skills within a year or two. We and they make such a big deal out of Minato's power and potential he had especially being the 4th Hokage, it would just take away from the Manga if it ends up being someone as simple as Obito.
Now here is a thought, what if Kagami was Obito's father so he knew the types of powers his son had and knew everything about his death and that would explain how Kagami knew all about Obito and the Leaf village.
It just makes more sense if Kagami is Tobi point blank.
If Tobi is Obito that means the 4th Hokage is weak! And now that I mention it, Tobi doesn't seem to be that powerful considering what we have seen from Madara, or Nagato, Orochimaru, Jiaraya, the members of the Akatski, the dead Kages of the Edo Tensi, and Kabuto or the old man Kage of the stone Onoki lets not forget Itachi. All Tobi really has been doing this entire time is spamming Kamu? and He has a mouth piece seeing that he has been manipulating everyone by lying. Tobi hasn't done anything explosive or really displayed any cleverly appealing skills to make you say Wow! that was pretty awesome, no black flames no elements no real Tijutsu no special bone/crystal/sand/glass/iron powers no nothing which has been boring. What happened to the good old days when we couldn't wait to see what new technique Orochimaru, his gang, or the Akatski was going to throw out?