I have used both these mics plenty. Neither one is perfect, both will need a little help. Personally, since you stated that it is to be used primarily for metal, I would actually go with the D112. The D6 has a very rich low end, and more solid mids, but the D112 will get you closer to that high frequency click you will probably want. The lows on the D112 will probably be just about right, the D6 you might even have to take some off of (depending on whether or not the guitars are drop tuned, 7 string etc....). The mids on the D6 sound better to me and are more usable, whereas on the D112 you will probably want to take the "boing" out by notching out some 250-630hz. Either mic however is capable of doing what you need, but I think the D112 won't need as much of a HF boost which is why I think the D112 may be the better bet for this particular use. A lot depends too on the kick drum itself of course and how it is tuned and played and even what heads are on it. As an all purpose kick drum mic, I would actually go with the D6, but neither mic would make me sad. The real difference between them to me is how I would process each, but both would almost always need it.
