Definitely go for NiMH or Lithium ion, don't touch NiCad (which are still
around, e.g. on cut price De Walt, I think).
I'd say a spare battery and a big charger so that they recharge in an hour
is more important than voltage. I've had a 14.4 volt Ryobi which has had
heavy use. I had to re-cell one battery last year but that has still been
reliable, nice to use, and good value. Makita always gets good reviews.
If I were in the trade I'd want pro models with hammer action. As a domestic
user I manage fine with mains hammer drills (a Ryobi again for normal, and a
budget SDS for big holes) and just use the cordless for drilling wood (up to
1 inch) and metal (up to about 1/4 inch) and driving screws.
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