A contactor is basically a long life relay.....since it has a more elaborate spring mechanism to make/break the contacts.
So if the lifetime with your relay is good enough, then stick with it, as a contactor is also more expensive.
if you can switch the relay when the loads are all off, then it will last much longer, and you wont need the contactor.......if there is much inductive current flow, then the relay will wear out significant quicker than contactor. Remember "breaking an inductive current" = "nono" to mother nature