Rationalising Death may be better if you haven't read Death Note; it's pretty good about explaining everything. As someone familiar with Death Note my feeling so far has been that Rationalising Death hasn't diverged enough; it sometimes feels like just rehashing the original. Not always, certainly, and I'm overall enjoying it, but that's seemed like the biggest flaw to me so far (admittedly, the author says divergence will increase as it goes along, and there are signs of that pattern).