This is a fun and interesting and very debatable topic!
Here’s my two cents and I’ll try to make it short:
First of all, I almost primarily flatpick guitar and a little bit of mandolin so that’s where my perspective will be coming from.
I think the most important thing about finding a pick you like is comfort. Next would be finding the most comfortable pick you like that gives you the tone you’re looking for. It also must slide off the strings smoothly so as not too sound scratchy. A smooth pick will give you more speed too. This is where pick material and shape of the point come in as well as bevel. Psheww, it gets complicated!
Most flatpickers today seem to like the beveled edge… such as Blue Chip’s speed bevel. They come in right hand or left hand depending on the angle of attack on the strings. Probably 90% or more bluegrass flatpickers today would go for the right hand speed bevel over the left. If you’re right handed and your angle of attack is the front of the pick (toward the headstock) is going down, you would want a right hand bevel, not left. Or, if you’re like me, I prefer no bevel at all which Blue Chip calls this their straight or round bevel. And then there’s the rounded edge like Kenny Smith uses. There’s also the real pointy tip like… uhhh, well… nobody uses!
My advice since this is so complex is to buy several different size, shape, thickness and points of cheap picks until you find one that works for you. Trying all these different picks is actually a fun process. Once you’ve narrowed it down to something you really like, then maybe try an expensive pick such as a Blue Chip. They are very expensive, but Ben and Jake are giving us the best price out there. If I didn’t already have two, I’d buy another from their store for sure! Again, I would wait until I had a good idea of what I wanted before making this purchase.
I will add that if you’re wanting a good Bluegrass tone with your guitar or mandolin, I would recommend thicknesses between 1.2mm to 1.5mm… at the very least 1.0mm. Even these can sound a little tinny depending on what it is.
Of all the enormous amounts of different picks I’ve tried over the last 40 years of still learning to play, I’ve settled on a Blue Chip STP-50 with no speed bevels. I’m not saying it’s the best pick out there, it’s just what I prefer over anything else I’ve ever tried. The next bestest pick that comes very close in tone and feel is a much cheaper pick, the Dunlop Primetone 1.4mm regular teardrop without the grips. You definitely should try these. I also have a real turtle pick (which most people call tortoise and that is wrong) and it honestly doesn’t sound that great. (not much volume compared to the BC).
It’s probably not exactly what you were looking for with your question, but the best way to get an answer to that question is to actually start test driving a whole mess of different picks. The one thing that’s for sure is the thinner the pick, the brighter the tone, the thicker the duller or warmer the tone. Everone would agree with that. You gotta find something in the middle that sounds right for one’s own taste.
As far as instrument relationships, I pretty much use the same pick for guitar and mando. I bet alot of other people do as well. But I’m sure there’s someone out there that is bashing me right now for this! Don’t care!
Anyway, I hope this was somewhat useful… and so much for keeping it short! Thanks for your question.