Not to harp on a point that's already been made, but if your primary interests are systemic reform and keeping people out of jail, then that sure sounds like a PD, not a prosecutor...
I think it's perfectly reasonable to be in favor of systemic reform and exercising discretion in a prosecutor's office, but most of a prosecutor's job is seeking the appropriate punishment for people who violate the law. And often, that punishment is jail time. I can't keep track of whether you're the original OP, but I feel like this is information that can be found out by talking to a few AUSAs in a district before going through the trouble of applying, interviewing, etc.
I'm not the original OP. It's like you guys are incapable of seeing nuance. What is the "appropriate punishment" for someone who violates the law? For someone who is caught selling cocaine and has a gun in his car? Your standard distribution of a controlled substance, which sends that person to jail for a while, or the standard distribution plus a tasty 18 USC 924 enhancement for an extra 5 (or even an extra 25, if the guy is a recidivist)? Guess what, between Holder and Sessions there's a a divergence of opinion as to which applies. Between different USAs in different districts I'm sure there's further divergences of opinion. If you don't let people who are more reform-oriented work as AUSAs, you're going to lose those nuances in prosecution and you're going to end up with a very repressive system.
This is just one example. It is very possible to go the AUSA route - ignoring for a moment the prestige-related motivations - thinking that you will be able to effect some
change, and that you will fine with the day-to-day realities of the job, only to realize after actually working there for a while that you can't take it. Kind of like all the PD washouts who come in as idealists and realize after a few months that a truckload of clients are guilty, lying assholes.