The benefit of using Next cruise is the reduced deposit as well as onboard credit. You can however, do both. You can book with Nextcruise (or purchase a next cruise certificate) and have the booking moved to you travel agent.
Something to keep in mind is that some of the larger travel agencies buy group space at a great rate and then these cabins are sold through thier various agents. If you book one of those cabins the reduced deposit is not valid and you have to pay the difference of the deposit.
I have actually decided to stop using Next cruise with Royal as I find there program doesn't really offer much benifit compared to other lines.