An SEO marketing agency is more of a full-service operation, often with multiple people responsible for the success of your campaign. SEO consultants can usually do all the same things as an agency, but they may deliver results much slower. This is because they would handle every aspect of your SEO strategy rather than splitting the work between multiple people. In either case, you’ll be working with SEO experts who should be able to provide you with advice, if not a fully fleshed-out marketing strategy.