I prefer Fenix.
I prefer fenix but nitecore seem better at innovation. Latest LEDs for example, little more adventurous in design. But some of the fenix classics like the tk75 and pd35 are hard to beat IMO.
Don’t think you would go too far wrong with either, if you have a list of wants, one brand might pick itself over the other anyway. If it’s compact 18650 your after, look at other options like zebralight for one.
Olight is another good Chinese brand, in fact there are many, Emisar another. Not all may offer the LED and colour temp you desire though. I don’t stick with one or two brands, have many on the go. Different UI, size/format and colour temps. Variety is the spice of life!…….apparently 
Fenix for me. Excellent build quality and zero problems. I’ve had problems with Nitecore in the past.
TK75 (2900 Lumen version) TK50, TK35, LD20, and LD15
I don’t like nitecore because they usually need button top cells, Also I feel that they focus too much in reaching “normal people” (the ones that would buy a light in a supermarket) and forget about enthusiasts.
I only have one Fenix (a TK16), it feels very solid, reliable and definitely a very good tool.
I like fenix better… i hope fenix will build a real thrower like a thrunite tn42… i think they will.