Both are fine. IQAS has been faster these days as they don't have a huge backlog. However, they have restricted the number of intakes in a day, and people have reported that they are not able to submit the application as the limit is reached pretty quickly. So you need check that. Otherwise go with CES.
I request someone to assure me if IQAS gave them a Masters before I progress with IQAS, and can anyone confirm if they have secured one from CES? I am torn between using IQAS and CES. I need help.