If you plan on buying a ticket on DL within the next 12 months then the take the voucher... its as good as $100 cash in your pocket.
Keep in mind DL vouchers CAN NOT be transferred, you must be the one traveling.
I would probably take the miles because I use them for summer travel to Europe. It aint easy, but I usually find 60k transatlantic awards every summer. In this regard, 10k miles is worth far more than a $100 voucher.