I agree, a journal article has much more prestige. It's a shame that that's how things are weighted, but that's how things are at the moment. Having said that, a published chapter in an edited book is much better than no publication at all. Some people also might publish their work in the book and then develop / extend the work / give it a different focus for a journal article. Personally I published my literature review (incl where I think research could/ should go in future) in an edited book and saved my major empirical findings for journal articles.