It's good experience, especially as a 2L. It may be worth checking the rules in the state you're looking to extern in; in a lot of states, 2L's can work as essentially junior prosecutors and try their own docket. PD externs can't do so due to the fiduciary relationship between the PD's and their clients, but since a DA's only "client" is the state, they can run the show.
Either way, you get a lot more hands-on experience, just because they're so under staffed. I wouldn't worry so much about the civil/criminal gap at this point (assuming you're a 1L) since you won't be picking up anything invaluable either way. Also look into clerking for a state trial court judge, then you get the criminal and civil sides of things, plus tons of experience.