In the case described, either lane would be proper. The right (curb) lane could be argued the safest option. When driving in the outside lane, the rule says signal in good time, then pull over to the desired roundabout exit as convenient. As in everything else, safe negotiation of roundabouts relies greatly on the other motorists understanding the rules and properly applying them - yielding to the motorist ahead who gives good warning of intent to change lanes in order to effect an exit of the roundabout.