My lights normally don't get any abuse, so I can only speak of the mechanical quality that I see and feel and the electronic quality I experience. I own both a few Dereelight and Nitecore.
Nitecore PD lights may be lights you need more to care about just because of the piston drive which may need some attention, a clicky switch normally doesn't, it works before it eventually fails. No clicky has failed on me yet though. I had two Nitecore with electronic problems, an Extreme GDP which began to flicker shortlyx after I got it, I returned it and got a new one which works. The second is the SR3, which has the strobe-bug, there's no remedy to this, you have to live with it.
The mechanical quality of my Nitecores it quite high, no obvious problem so far. The finish is excellent, the thickness is OK too, threads usually very smooth, lenses may have a diamond coating on the outside, which is outstanding.
As for the Dereelights I have not much to report. I just received some XP-G and they are great! One of their advantages is, that they don't change the design much and/or often, so the lights rely on a tested and solid, rugged construction. The finish is flawless (I don't use an electron microscope though...), threads are OK. My DBS I received this week has a foggy lens, some errors in the coating I suppose as I can't wipe it off. I dropped them a mail and a new lens is on its way to me. Excellent CS!!! :thumbsup:
A light should light I suppose after all and that means beam quality. There's not much to say, each light is made for a special use and its beam should be adapted for this use according to the output of the emitter. My Nitecore are up to this task, though some are too throwy for me, like the EZ123 or the SR3. That's more or less a personal consideration. On the Dereelights you can mostly choose between SMO or OP reflector (or have both!) and mine are all perfect as I want them too, except the CL1H V4, which I feel also too throwy, but I got it an aspherical head, now it's perfect!
Last but not least, all Dereelights now sport the latest emitters available, the Nitecore don't at this moment. The Dereelights are upgradable, that's their strongest point and you can have most in neutral white (also as an upgrade). Now choose which one you buy first! lovecpf