This new article compares MTurk workers, Microworkers and Clickworkers. Here’s a summary (please note: verify information in this article for yourself because I’m not trusting this information completely).
Mturk: the article says workers must be based in the US (false) and that the terms of service are strict (debatable). There’s no ‘vetting’ of tasks (the article sees that as a benefit) with no minimum wage.
Microworkers: the article says tasks are easy to set up and a variety of tasks are acceptable. It takes up to 48 hours for tasks to be approved and there is a (low) minimum wage.
Clickworkers: the article says that this site charges requesters a set-up fee so that gets rid of a lot of startups (and probably most academic research). Only 10-15% of workers are US based.
I’ve never worked with either of these sites (Microworkers or Clickworkers) but if you have and if you’d like to share your experiences, please post in the comments!.