Tone is in the fingers, as they use the pick, so heavy/light/thin/thick/tortex/plastic/tortoise all have so many permutations (particularly tortoise, since, as a natural product, there's lots of variation in strength, density, stiffness, etc., even among picks from the same species). Folks are even trying cow hoof, deer antler, and non-threatened tortoise species.
I find that I use my tortoise picks only on mandolin, and Fender mediums for guitar. But I'm not a Norman Blake/Doc Watson/ Dan Crary flatpicker (and, parenthetically, neither was Merle Travis, who used thumb and fingers, not a flatpick, so far as I know), so YMMV ("Your mileage...."). I know that if you want to do the single-note style of playing, rather than the full-strum rock-god rhythm style, you need something from medium on "up." Those thins just don't make it.
Of course, on electrics, it doesn't matter so much, it seems.