I used to use instants exclusively since they tasted better, had no prep time involved, no room for error and worked well with less side effects. They got too expensive over time, the normal 2 tsp recommendation was too little to feel any effects. I would need about 2 tbsp with most, per "shell" and would use 3 or 4 of them. The effects come on faster and stronger than medium grind but don't last as long either. Not really good for getting a krunk effect but definitely better for being out and functional while using it.
I went back to medium grind due to cost savings, the vast variety and the muscle relaxation is much stronger. I feel like instants pack more of a kavain punch. I've mostly used GHK Moi and Mahakea but also like BKH green quite a bit. I used to use a lot of Shaman and Fire Island but the Fire Island hasn't been as strong as it was a few years ago. Cream of Vanuatu was also great. The only thing I didn't like was the strength batch to batch was always different (some weaker/stronger than others) but I've found the same to be true for medium grind.