The plot of Days has nothing to do with Death Note canon and several characters are OOC, but it's fun anyway and the OOC-ness is intentional. L's new persona is particularly hilarious. Nights is Days told from Mello's POV instead of Matt's, and in some ways it's the better of the two. Mello is awesomely snarky, and the Mello/Matt hints are more obvious from his POV.
FanficRecs sauronlulz' review
The plot of Days has nothing to do with Death Note canon and several characters are OOC, but it's fun anyway and the OOC-ness is intentional. L's new persona is particularly hilarious. Nights is Days told from Mello's POV instead of Matt's, and in some ways it's the better of the two. Mello is awesomely snarky, and the Mello/Matt hints are more obvious from his POV.