What's preferable, disk, tape or both for DR?

A disk MIRROR with a hot DR site is preferable, but very few people can afford that. As long as we are talking...

a RESTORE, the answer will depend on your environment. What is preferable is what works for YOU.

Tape can be faster and less expensive for massive bulk restores. Disk can be faster and less expensive for smaller bulk restores and SOME larger restores. Tape is easier to ship off-site for DR purposes, but disk can easily be mirrored off-site without moving anything. Each company must do a cost-benefit analysis on what works for them.