No recommendations here, but I usually sail Carnival and love it. I've never been on
RCCL, so don't feel qualified to answer that question.
If you have at least 5 cabins to fill, I suggest that once you decide on your ship, you call the group dept at the cruise line for special pricing and group perks.
After you get the price, etc from the cruise line, submit your info to
www.cruisecompete.com and get pricing from several different TAs. Even if you don't have 5 cabins, you should still submit your info to Cruise Compete. I've used them for both group bookings and a cruise just for my husband and me.
I've organized three groups and have yet to use the cruise line to book. We also have always gotten some nice perks (a cocktail party, photo coupons, chocolate dipped strawberries in our rooms, bottles of wine and always at least $100 in on-board credit to use).
Good luck with you planning!