Overall, my general opinion is that for most applications, I'm happy to use thrust curve to help me find engines to test that suit my goals. Otherwise, I just bring up the casing sizes I have and look at the available loads. I care relatively little if my engines have a red or green flame, but I think there is a major difference between Dark Matter and Metalstorm. Dark Matter has titanium sparks and a lovely black smoke. It has trash ISP, but I adore it anyways.
Metalstorm, I just don't like it in comparison. It has sparks, decent isp, but white smoke and IMHO, the wow factor is just gone. I've looked at several sparky motors and been dissapointed to find out that they are Metalstorm, not Dark Matter, so I'd love to have an option to search for them in particular.
Video comparison:
Metalstorm Dark Matter Big Dark Matter
I actually find even the smaller dark matter motors to be more impressive than the larger metalstorm sizes. It's just my personal taste. The other big issue is that the dark matter ones are often labeled metalstorm dark matter, which can lead to confusion. I like
Buy Rocket Motors ability to search by prop type, but if that is too much work, just the sparkies would be nice.