Ambiguous endings, as long as they're consistent with everything that came before, are perfectly valid. Most of the plot in Dark Souls is arrived at through exploration and consideration. Both endings carry with them enormous implications. If you choose to sacrifice yourself, are you really doing the right thing? There's various bits throughout the game that suggest that you aren't, and that you are, in fact, just delaying the inevitable and forcing future 'heroes' to sacrifice themselves as well. And if you deny the sacrifice, the implications are totally unknown, since you're entering a new age that has never been seen before. Just because it takes a little thought to arrive at the conclusions that the game wants you to make does not mean that the endings are bad.