The best way to find out is to look at the benefits list for each organization you're considering. You will find that InterNACHI always:
Has the most membership benefits (exclusive tools, discounts, etc)
Has many more free continuing education hours available to members
Is always helpful and responsive—our founder and CEO both personally answer member questions every day
Has the largest community of inspectors
In the end it is a personal decision, but with InterNACHI you'll always get more bang for your buck.