I suggest you definitely go with elance. You will have to deal with non-native speakers messaging you, but you can just screen them.
I personally use elance a ton, and like the setup they have. Built in escrow services, tests for freelance professionals to prove their knowledge, and a useful overall job system.
For writing jobs, I usually stick with individuals and not groups. Also, I look for people like myself to hire. Basically they are in college(in the U.S.), and are looking to make some money writing. These types usually work very efficiently and with a good deal of skill. Everything else can be explained to them quite easily.
I suggest you stay away from fiverr if you want real writing. You will only get non-native speakers, children, or just incompetent fools.
I'd tell you to PM me, but I really don't like content writing anymore. Way too boring, and not enough pay. All the international folks have made it way too hard to make any money. Much easier selling papers to other students.
Best of luck.
