Generally a Hearing Aid Specialist is not an audiologist. They have a state license to dispense hearing aids. This means they have (depending on the state) passed a written and practical exam that shows proficiency in and understanding of their field, and sometimes some kind of supervised apprenticeship has taken place.
Although it varies by locale the term senior usually applies to a dispenser who has been fully licensed for at least two years and is qualified to supervise and train an apprentice.
As someone else has already explained an audiologist usually has a masters degree in audiology or even an AuD which is like a PhD but without having to write a dissertation, and is specific to audiology.
As with any field there are good professionals and bad professionals. Competent and incompetent. In my job I supervise and train both HIS and AuD employees, and there is a variance in skill and competency across both qualifications.
In my personal opinion, an AuD is likely to have a broader education covering more unusual kinds of hearing loss, children, and profoundly deaf people. A HIS probably has plenty of experience helping adults with more common types of loss (age related, noise exposure etc).
As to who is better, it is down to the individual. I’ve experienced good and bad from both sides of the field.
Consider having two or three free tests, and maybe one from a ‘real’ audiologist. Use your common sense to compare the work they put in, and get a feel for who you trust to help you.