My advice to everyone is the same ... Before you ask a programmer for a quote, make sure to write down the strategy in as much details as possible. Obviously exclude the secret sauce. This helps in two ways: 1 you clearly understand what you need. 2 it's easier to estimate the scope of work.
I just looked again at both sites ... (only the first page). Saw a variety of countries represented. These sites are more popular in foreign countries since the coders there can essentially underbid U.S. coders due to their lower cost-of-living than the U.S., imo. That said, you can pick a coder from any country you wish. As others have stated, you want to be logical, clear and detailed in communicating what you need to the site you pick. Obviously, you need a coder that can understand logical, clear and detailed English, or the language of your choice. Read all instructions, terms, and disclaimers on the site. Research the site further, and the coders you're considering. Due diligence.