Well it's largely down to personal taste - I think they are both superb bits of kit and it's the D10 that is in my pocket right now. When I say I prefer the NDI, it's for quite subtle differences. I'll try and explain...
I like the piston switching system on the D10, it feels robust and well machined, but for me it has some shortcomings over a regular clickie switch.
With a regular clickie, you can have momentary or lock on/off just by varying your thumb pressure. A light press for momentary, a firm press for click on. It's simple, intuitive and I like it. With the PD system, you get lock on - if you want momentary, you have to turn the light on, rotate the bezel (2 hands required) till it goes off, then you have momentary. But now you have lost lock on, unless you turn the bezel back - in which case you loose momentary again. I much prefer being able to choose between momentary and click on just by varying my thumb pressure, especially if I only have one hand free to hold the torch. NDI wins for me.
Variable output: Both the NDI and D10 have variable output and remember the last used setting. However the NDI allows you to select between full bright and memory by a slight turn of the bezel. This means you can select which one you want
before you turn the torch on. With the D10, if you last used minimum, that what you get when you turn the torch on. If you last used max, then that's what you get when you turn the torch on. You can
only vary the brightness with the torch turned on. This can bugger your night vision if your last remembered setting was maximum and all you want is minimum, as you have to fire up the torch and
then dim it down. NDI wins again for me.
The machining and construction on both torches is superb. I very much like the ability to stand the D10 on it's tail and this really should be a feature of the NDI also. Here's hoping they release an alternate tailcap which does this in the future. I also like the anti-roll flats machined into the NDI - but I'm not a big fan of the crenelated bezel.
The D10 has no strobe mode. So what I hear you say - well (dubious) tactical applications aside, this can be a useful signaling feature, especially if used with one of these...
I have the red and white Fenix diffuser and while the fit on the Nitecores isn't perfect, they are usable. They are a great idea. The white diffuser turns the D10/NDI into something approximating a small fluorescent area light - very cool. Use the red diffuser as a nightvision preserving "submarine" light. Stick the NDI onto strobe and use the red diffuser and you have a very useful, rapidly flashing red signal wand. Nitecore should think about making diffusers like this specifically to fit the D10/NDI IMO - they are great.
NDI on strobe with red fenix diffuser...
http://www.youtube.com/watch?v=5RWI5cXvDYM
Something interesting I just noted, I ran a battery (energizer lithium) flat in the D10 to test runtime and after it was flat, I stuck it in a Zebralight H50 to see if there was any juice left in it. Nada. Then I put it in the NDI and to my surprise, I got another 10 minutes of useful light. Dunno what's going on there, but the NDI wins again.
I like em both, but for me, the NDI is the
slightly more versatile and useful tool. The D10 is fine as a general duty utility torch, but for camping and such, the NDI's simpler, more versatile, more intuitive interface is better. But both are superb, either would be fine and both are better than anything I've seen from anyone else.
I have to stress that these are very small points, in practice there is not that much to choose between em, they are very similar torches.
Here are a couple of superb youtube reviews...
Nitcore D10:
http://www.youtube.com/watch?v=irqhuV8Xo_A
http://www.youtube.com/watch?v=ruYgBHzxSVI&feature=related
Nitecore Defender Infinity:
http://www.youtube.com/watch?v=daFqQjRYgMs
