Dear Mr. Simon,
CMA course is not a substitute for CA course. Choosing a course depends upon what area you would like to specialize. Finance is like the Indian ocean; it is insanely vast and connects a large number of countries.
CA, CMA, CS, CFA, MBA (finance) etc are courses related to this discipline offering various specialization.
As regards scope and pay, simply because you have a diploma such as CA or CMA or CS will not automatically traject you to fame and a fat pay-pack. Clearing any of the professional courses for that matter is a task that requires immense hardwork, motivation and dedication. But the real challenge starts after that; to stay abreasst of the changes and to continually strive for professional excellence in the chosen sphere of specialization. Corporate world is extremely competitive nowadays.
There used to be a time, I beleive when membership in a professional body was considered sufficient for a decent career path. Nowadays, students are coming out into the job market with multiple Degrees and diplomas. It doesn't mean that all such people stand better chances. Upon my personal discussion with the partner of a leading accounting firm, the partner had remarked that he personally would'nt hire a fellow who has multiple degrees as according to him, it shows a lack of direction. But again, this view is subjective and varies from person to person.
Finally, to conclude and answer your query, Go for CMA if you are interested in specializing in Cost & Management Accounting. There is ample scope for GOOD CMA just as there is always scope for GOOD doctors or engineers though you may find thousands of doctors and engineers without any professionally or economically rewarding jobs.
The reason why you are opting for CMA over CA is because some ill-informed friend or relative of yours told you that CMA is easier to study / pass, dont be fooled with that impression (speaking from personal experience)
Wish you a successfull and prosperous future ahead !