You keep the rights to your photos no matter what. You are the photographer, and, while a company can offer to buy exclusive rights to your photo(lots of money to do so), this hardly ever happens.
Rights managed- I do this, as do many photographers who feel that royalty free is not as good for photographers in the long run. In the rights managed category, your images will have a specified usage ie: print run #, online or in magazine, book, etc., how big the photo will be in a publication, etc. Alamy then prices the image based on the types of usage the client selects. This can also mean lower prices if the client wants small inside photos, low print runs, etc.
This is why some turn to Royalty Free- client can buy the photo, use it however they want for however long they want for a fixed price based on size. For larger images this usually means more money, but less repeat sales from clients.
From my personal experience, you tend to sell more at lower prices in RM, and less for more in RF, but the numbers end up favoring RM most of the time. Novel use licenses will most assuredly lower prices for clients that use it, but it will bring in more business than you have otherwise, so I am signed up.
In the end it's your decision, and I'm sure that other photographers will have different experiences. Hope this helped though.