I've been thinking about purchasing a used D10 or D20.
The D20 is superior in every way except aperture. I doubt that underexposure will be a problem with lightning photos. A bigger issue is the field of view. The D20 has a wider field of view, so you'll see more sky and thus, more lightning bolts. You may even want to try video mode for lightning. The D20 has HD video. The GPS may be useful too.
I haven't tried the D10, but the D20 is great in a storm. I tried putting RainX on the D20 last night, and did a time lapse in a nasty storm. One of the local TV stations used it in their news broadcast today:
http://www.youtube.com/watch?v=ISenBA_T2r0#wsLens flare and reflections with the sun in the frame are a problem for any camera. The D20 is better than most, because of the simpler lens. The super-zoom SX50 is horrible, and the G1X is a little better than the D20.
I put the D20, G1X, and SX50 on a camera bar for these three time lapses, with the sun in the frame at the very beginning, if you'd like to see the lens flare/reflections:
D20:
http://www.youtube.com/watch?v=RvctLt6p-Ro#wsG1X:
http://www.youtube.com/watch?v=tDtN6FqtgIg#wsSX50:
http://www.youtube.com/watch?v=80_k6QMq3ao#wsI haven't noticed the sharpness problem at wide angle, but I haven't looked closely. I assume it comes from the lens distortion correction when creating the jpg image from raw.
We don't get much lightning where I live, but I have tried to capture meteors a few times. For that, I want the widest field of view possible. I assume you will too, since you never know where the lightning will be.