In many jurisdictions it does help to have an alcohol abuse evaluation prior to beginning of the court process, in other is makes no difference (the court may order it if you end up with a plea arrangement or are convicted after trial). You should discuss your situation with an attorney in the area where the incident occurred and who is familiar with the practices of the local District Attorney's office.
The question you posed in your caption was the difference between a DWI (Driving while Intoxicated) and DWAI. (Driving while Ability Impaired). First, the difference is that the first charge is in regards to the consumption of alcohol and the second is in regards to the use of drugs. Both are misdemenaor charges and sanctions include fines, probation or incarceration.
In regards to an evaluation, many counties want a governmental run program or one that is sanctioned by a governmental entity. OASAS (Office of Alcoholism and Substenace Abuse Sevices) providers are usually accepted and a list of such providers can be found at the office's website.
In most cases, I tell my clients to wait to obtain an evaluation until I have spoken to the assistant district attorney and have an idea what possible plea is available or if the matter is going to be tried. Please note that many insurances will pay for the evauluation but not all.
If you meant DWAI the violation Driving While Ability Impaired, then there is a big difference. DWI is a misdemeanor leaving you with a permanent criminal record, with up to one year in jail, three years probation, $1,000 in fines, and a 6 month revocation of your license. Also if you plead guilty or are convicted of DWI, and you get re-arrested for DWI in the next ten years, that charge would be a felony. An impaired is a traffic violation and not a crime, punishable by a maximum of 15 days in jail, $500 maximum in fines and a 90 day suspension of your license. So you see there are big differences.
As to the alcohol evaluation, it depends on the county where you got arrested. In Nassau for example, it is mandatory. So I would make sure you have competent counsel in your area who has experience dealing with DWI to get the best result possible.