So far I received two interviews, and hypothetically speaking if I were to receive both offers, which one would be the ideal in terms of eventually bringing back the knowledge to my own sites?

Both positions are for search engine marketing specialist, which refers to organic and paid search positions.

One company, is a top 5 advertising agency, with offices around the world. I'd be a small fish in a big pond, but have the advantages of large scale projects and challenges, and the resources to solve them. Perhaps I could transfer elsewhere in the world, and enjoy travelling and still hold a steady job. The downside is the same as working for any big company. Very rigid structure, not a lot of mobility, everyone sort of works on a very specific cog of a huge machine. Now perhaps these things are alleviate somehow by it being an advertising agency, with all the stereotypical fun/creative/laid back energy. I do not know.

The other company, is a local startup, of maybe 100 or so employees, it deals exclusively with hotel/motel organic and paid SE marketing. I can see a lot of potential here, 1) because hotels/vacation packages are a highly competitive and specialized niche, I'd learn very specific skills that I am sure I could then port easily to less competitive areas. I could also grow inside the company probably much faster and/or assume different roles. Internet marketing is all they do, so I assume I'd become very good at it. The downside, is that hotels is everything that we would do, from now to 2-3 years from now. Don't get me wrong, I'd love to be able to say down the line that I work for the top internet marketing company, and that we manage the top 10 hotel chain internet budgets, but I'm afraid it may be too restrictive?

Thoughts? opinions?