It might depend on what you want to have translated. If it is a technical manual, or a brochure, or something similar, an agency is an obvious choice. If it is a literary work, or anything that uses language in a subtle or expressive way, it would probably be best to opt for a choice that gives you the opportunity to discuss meanings and interpretations with whoever is doing the translation. In some cases you may have to settle for a less than satisfactory version, but one that comes somewhere near the original meaning. A good personal interaction with the translator is very useful in that case. To a certain extent, the translator will “take over” the work and make it their own – it becomes a co-operative enterprise. I used to work with someone who translated Eliot’s Wasteland (working with the author himself). A good translator will have a good deal of tact, in all sorts of ways.
Google Translator is my best friend. 