Perhaps the difference that strikes me immediately is the issue of mediumship. Tarot, as powerful as I think the little cards are, feels more like an extension of your own awareness, talents, and knowledge- however hidden it might be. In my mind, it's more like equipment for toning your psychic muscles, so to speak.
The Ouija boards/spirit boards/etc. I feel require a degree or element of mediumship that I'm not willing, or too frightened, to offer or experience. My personal belief is is that any spirit or deceased loved one with an important message to share, would find a different means of passing it to me. I think they would come in dreams, or even less subtle ways.
That said, there is a Ouija board that I would love to own, for the simple sake of collecting, but not for use. Some of them can be quite beautiful.