Mostly deal size and process. Business brokers generally handle smaller main-street transactions and list them; M&A advisors run confidential, targeted processes for lower-middle-market companies and negotiate more complex structures.
More detail
For a manufacturing company in the $5M to $20M range, the practical differences are how buyers are found and how the process is run. A listing approach reaches whoever is browsing. A targeted process identifies specific strategic and financial buyers, approaches them confidentially, and creates competition among a qualified group. Deal structures at this size also involve working capital mechanics, earnouts, rollover equity, and quality-of-earnings review, which is specialist territory. Credentials vary across the field, and the meaningful questions for any advisor are how many transactions they have closed in your sector and size range, who specifically will run your process, and whether they will give you references from sellers whose deals did not close.