She would need to be evaluated in order to determine eligibility for an IEP or a 504 plan.
To be eligible for special education, she must display a processing deficit and achievement that is at least 1.5 standard deviations (about 22.5 points) below her assessed cognitive score. The combination of these two things establishes the presence of a specific learning disability. To determine a language impairment,(different category of eligibility) her language scores must be two standard deviations below the norm (30 points) with the norm being a score of 100.
504 is a little more inclusive because it is a nondiscrimination law under ADA rather than a mandate for services under IDEA. (Welcome to the alphabet soup of government regulation!) She would qualify for a 504 plan if she has a documented disability and requires accommodations and modifications to make progress. She does not have to show a discrepancy between achievement and potential achievement.
You should know that most states do not recognize CAPD as a specific disability in and of itself, it is a relatively new explanation and the regs are always slow to catch up, so your daughter would still need to do the normal eligibility procedures for your area, under the law.
You may have had this information already- or I may have completely confused you:), but it sounded like people were giving you the impression that you had a choice between IEP and 504- you do to a certain extent- but there are eligibility gates to be hopped first and if you don't then the choices are different.
Robin