Context context context
I don't think they are the same word. I also think they are different. Depends on the context and who is saying it and why.
I don't say it regularly, but I also don't assume that my friends or people I personally know are too stupid or fragile to hear the word if it's necessary to conversation. If I'm with my friends and we're talking about a lyrics to a song or something I'm not going to say "n-word" instead of "nigga"
I have friends of mixed races, sexual orientation and other such things. In my circle we like to actually talk about shit, and not keep each other at a words distance.
I think it's fine for a black person to say if they'd like to say it. I can understand the philosophy that "owning" the word saps it from some of it's offensiveness, but I also understand the viewpoint that the "softness" of the word "nigga" vs. the visceral harshness of "nigger" makes it run the risk of letting young white people feel comfortable throwing it around... however, if some future generation has no more hang-ups about the word, I'm not going to crawl out of my grave and tell them to not use it either.
So yeah, depends on the situation. See also: nearly any word in the english language
There are situations where calling someone "stupid" is more offensive than situations where some friend is lovingly calling his friend a "nigga"
So context context context