Rocket and nfps are both the OG's, so to speak. Iks66 is kinda the new kid on the block. All provide access to the same service, so at the end of the day it's 6 of one or a half dozen of the other. Only real difference you'll ever potentially see is in customer service should you ever need to contact them, which you shouldn't. Aside from that, iks66 offers monthly subs where nfps and rocket are yearly only.
Pretty sure this particular forum is offered by the same folks who run ftabeta, the official help forum for iks rocket, which explains the staff loyalty to rocket in particular.