I have been childminding for 18 years and always insured with Morton Michel.
I think you need to think about 2 different things - insurance and membership.
If you are a member of NCMA you are a member of a national association which supports childminders. You can then buy your insurance via NCMA. You must use their paperwork if you buy their insurance.
If you buy your insurance from specialist early years insurer Morton Michel you don't need to use their paperwork you can design your own.
Both companies offer a similar range of added extras free of charge.
I hope this helps you decide