Two things can happen. You ride the tandem a lot or you don't.
If the tandem gets a lot of use then I suggest no matter which tandem you buy now you will most likely want a newer better one. Think about how many singles you have owned and the likelihood that you would have stopped at the first one. As a result I suggest you go with the used bike option. It should resell without much loss to you and you will know more about what you want in a tandem when you upgrade to a new tandem.
On the other hand if you buy it and do not ride it a lot you will be glad you went the lost cost route.