I've worked on both sides, both the recruitment agency side and working as a temp so a little advise for you in these type of situations. From the recruitment point of view clients (companies) override your importance anytime, clients also have an annoying habit of changing their requirements, timelines and recruitment needs at the last minute. As an employee your best bet is to take up a job that is offered you the first as you can never be sure of will the other job pan through. If it does, and you really really want it, you can always ditch the first one and take the latter offer. Will piss the other rec agency off and they might remove you from their list, but you would be left better anyway at the end.