ACNP- hospital in-patients, specialty clinics/offices (adults only)
ANP- specialty clinics/offices (adults only)
FNP- clinics/offices (children and adults)
It all depends on the acuity that you want to treat.
Vanderbilt has an ER-NP program, consisting of the education of an ACNP and FNP
Best bet is to get both certifications for the ER, otherwise if a peds pt comes in the ER and you only have an ACNP, you would have to leave your scope of practice to treat that kid. This limits you in your opportunities to function in the ER and thus, with only an ACNP, makes you fairly unmarketable in the ER.
I am relocating my family to Nashville for Vanderbilt's ACNP program.