Hokay, I've gotten to try both now and I'll do my best to give a good comparison. I've already described the feeling of Mo'i, as have others here, so I'll just try to stick to a comparison between the two.
The instant I feel set in more quickly than the med. grind, most likely due to taking all of the contents into the body and not straining it out. The med. grind took a bit longer, say about 35-45 minutes, to set in. The instant had a much "sharper" feeling, like the effect was juuust a bit stronger, and kind of grabbed my attention, like "hey, this is happening right now." The med. grind had the same overall effects but it has a wider range, so to speak. Where the effect of the instant was easily discernible, the effect of the med. grind kind of came and went over the course of a few hours. Sometimes it would rise, then fade away for a few minutes, rise again, etc... The instant didn't do that, it just stayed with me.
I kind of enjoy the traditional prep, but of course it takes time, and it's nice to have a sink handy for washing up. I just made 3 tbsp. of the med grind for my Sunday morning, kneaded for about 25 minutes, then did a second wash in a little less water and mixed the two together. I'm noticing that 3 tbsp. of med. grind has about the same effect as 1.5 tbsp. of the instant, which makes sense since
@Gourmet Hawaiian Kava says the makas are removed in the instant, and from what I've seen that's about 50% of the weight, maybe a little less.
Important note: neither the instant nor the med. grind had any negative gastrointestinal effects on me at all. However when I took micronized Mahakea, I noticed just a tiny bit of GI upset (very minimal and I wouldn't have noticed had I not compared to instant), perhaps due to the slightly larger particle size vs. instant?
Overall, bang for buck, I think they're actually about the same, perhaps a little more "buck" for the instant, however the same effects can be felt with less of the material AND less of the prep time. It's just harder on the wallet to take a bunch of it. Next time I have some extra $$ I'll be ordering more of the instant, but for the time being and I think for a regular-basis kind of thing, the med. grind is more affordable, and that's a nice trade-off for me despite the prep time, which is really only about 30 minutes. Not very much time out of a day.
Hope that helps anyone who's wondering about the differences between the two.
Edit: just wanted to add that I prepared the med. grind by kneading it in a cotton muslin bag in water. I think this afternoon I'll try the blender method and see how that compares. Maybe blend for about 4 minutes on high then strain through my Aeropress with the muslin as the filter instead of the paper coffee filter. I'll report back on how that goes...