Back Carriers

Hi all, I've never used one of these before, but we are off on hols next week and taking a single buggy (or the luna) Terry asked if Luka could still go in the baby bjorn carrier I think he's far too heavy now. Has anyone used or can recommend a good back carrier we may use it for either child and will be going on long walks so just wondered if its something worth getting as I know they are quite pricey?