Honestly, I think scouts spend more time in the field. They go for long walks, go into the weeds to build campsite and structure. Eat outfield with the bugs and creepy crawlies. Their advantage is that they do that VERY often.
NCC cadets are more or less seen around school doing sai kang. ( I suppose thats one preparation for NS also
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Learning to shoot an M-16 is no big advantage. Anyone shoot a weapon. BMTs BTP adequately prepares a recruit even if he has no firearms experience. Most non NCC cadets are able to shoot with above average results.
So if you want a CCA that prepares you for NS. Join scouts. The conditioning that it provides to field environment is invaluable.