For me, there is 2 types of gripping.
Lose and tight (don't you think elsewhere)
So yes when i am strumming and i dun require any intense strumming sound, lose grip applies automatically. When it comes to picking, most of the time i am picking with tight grip. Oh but when it comes to playing things like sweeping, it's normally lose rather then tight.
Anyway i am using Planet Waves .91mm injection molded picks. Not very durable but most of the time i pick light, so it's ok. Anything thinner will result in floppy picks that don't response to you when you pick fast. Anything thicker will not really strum smoothly even with a loose grip.
I've also tried V picks before and i do like it.You'll really hear the tone difference and the smoothness each time you hit the string.(the edge are unlike normal picks, they are beveled) But it's not suitable for long hours playing to me as the pick is seriously too thick for my fingers to handle. If you are conscious about tone, give that a try.