I think people who are born-in and then leave are more likely to become non-believers. If you grow up to develop the right kind of reasoning skills then you're unlikely to simply leave and then join another group or subscribe to another theological belief. The logical thinking that leads you out of the organisation usually leads to atheism, ultimately. That was the case for me, as a born-in who never really believed whole-heartedly.
Those who join the JWs as adults obviously lack the same innate scepticism of religion, as it would prevent them from joining in the first place. So I think those who leave having not been born-in are often likely to move on to some other religion or at least keep believing in god.