I think both same service you can hire writer bottom line ,.. but may have competitive prices,,different skilled writers and full writers have more cheap rates i think but they all write articles or content once we hire them right hope you got already
I have only used iWriter from the two so can't really go in-depth but one thing that sticks out is that on FullWriter you seem to have the option to also purchase prewritten content while in iWriter you hire the writer to write the content for you specifically. Not sure if you are a freelancer or a client that wants to have their content written but in both cases you can also try different sites like oDesk, eLance, Freelancer etc.