Both are recognized in Sri Lanka. CALSDA has a larger scope, however there's no information found on International Accreditation for both of them. Diploma – Speech & Drama for Primary Teaching - DIPCALSDA (SDPT) would be a better start for a preschool teacher.

IWMS is the best. I am not saying just because I'm a teacher there. It's the world recognized and the best qualified teachers are found there. I advice you to visit the institute and get more information and see for yourself how it goes in there and your second question in not so clear to me but I guess you wanted to now what I recommend for a beginner in Elocution. Actually if it's to teach elocution you need to have atleast followed the higher grades but if you are fluent in English and have good record you may start from some where in the middle. I cannot give you the exact grade because I have no clue neither about your knowledge nor capabilities. So if you start from somewhere in the middle I advice you to start teaching with the little ones not more than grade three because the grades are harder to teach grade three above and if you do start from some where closer to the end you may go till grade 11 but I advice you to go get your licentiate while teaching the the little ones.
Best of luck!

IWMS exams are high in demand by students and that's probably because they have been there for a pretty long time in the country. CALSDA offers a good start for teachers who are interested in pursuing a career in teaching English. That being said, the Trinity College Speech and Drama Exams are way more recognized than any of the IWMS and CALSDA teaching diplomas. But If you are sticking to teach Spoken English, I would personally recommend CALSDA.

CALSDA is better to start the career not only as a Pre School Teacher. It is accepted for higher grades too