If you're planning on ever editing the video, then storing on DV tapes is the way to go. If you've got hours upon hours of video which you never intend on editing, only viewing, then DVD is the most cost effective.
which will last longer? The effective life of both will be dictated by the life of the both the storage medium itself and the technology used to play. It's likely that a DV tape will be more robust in terms of storage, but you're reliant on a DV camera being available in ten or so years - and if you;'ve recorded in LP, you may well need the exact same machine. chances are you're DV camera will expure withn the next few years. On the other hand, your next Blu Ray or HD DVD player will be backward compatible.
My advice? Never record over your original DV tapes and create DVDs of your edited material. As soon as a new medium of storage comes along, transfer your DVDs to this medium.