Thanks to the kindly reader who asked me to address the interaction between state and federal law, from my perspective as a prosecutor.
First, the law: you can be prosecuted for the same offense by the state and by the feds. Yep, I know, seems a little unfair, doesn't it? Smacks of that double jeopardy thing. But no, turns out that because the state and federal governments are separate sovereign entities, double jeopardy doesn't apply. (Peruse the case of U.S. v. Koon, if you wish to explore the logic.)
Now then, no need to get your knickers in a twist: in practice it rarely happens.
What happens is that the feds cherry-pick the easiest cases to prove. Aw shucks, that sounds churlish but there's some truth to it, and I don't mind in the slightest. Really. I always think of them as very serious folks in dark suits conferring with equally serious, sunglass-wearing FBI agents. So it makes sense they get the more serious cases.
Anyway, the overlap comes in two types of cases: drugs and guns.
Drugs: the feds are interested in larger amounts of drugs, that's pretty much the decider, frankly. Pretty straightforward.
Guns: I'm not totally sure of the honey that draws the feds in for these cases. Gun cases are usually "felon in possession" cases. State and federal statutes provide for this offense and whether or not we keep a case or it goes fed is . . . not clear to me.
So what's the difference? Well, federal penalties tend to be a lot stiffer, and whatever sentence you get, you tend to serve almost all of it. Not big on parole, those guys.
Interestingly (to some, I hope) if you are facing both a state and federal sentence, make sure you get your federal one handed down first. State sentences almost always get run concurrently (at the same time) whereas if you get your state sentence first, the feds will wait for you to finish it before they come get you for your federal time.
On the other hand, I've heard the food is better in the federal pen. Let's hope I never find out.