So I have this story regarding a mass murder mystery in an urban fantasy setting but can't decide whether it's a regular noir film or if I want to have a bit more fun with the look and head into neo noir territory. It's a baby idea so I don't want to say too much least it fizzle out but it's got a heavy magic element and involves spirits as companions.
"If I want to have a bit more FUN with the look" (Caps mine) I'm go with the fun option as it would probably feel more "alive" when you read it merely because you're having fun with the story.
From my understanding noir means it's in a 1940s style setting. Rotary phones, no computers, etc. Neo noir is a modern setting. Is this what you mean? There's this urban fantasy I read, Kate Daniels series by Illona Andrews. There are times if day when magic works and technology doesn't and vice versa. I always thought the concept was interesting.
Noir films are characterised by high contrast lighting (ironically called low-key lighting) and the plot, often a detective story. Because this is a visual style choice the plot bit doesn't much matter, also given that it's a murder mystery anyway. Neo Noir is a more recent adaption of the style, these days a lot of neo-noir films use coloured lights, like neon lighting instead of traditional white or yellow light. Both are sort of considered genres. They're a grey area but the definitions are like that of genres, they don't really change especially given that the rules needed to classify a story as such are based on the plot and visual aspects rather than the usual genre stuff which is more based on motifs and themes of the stories. The time period has no baring on if it's noir or neo noir. That classification is reliant on the story and the style of the visuals.