100 million years past as I have always been more interested in Prehistory then the future.
To me creature like Dinosaurs and the other forms of life that were around at that time and even the natural world that these creatures lived in is far more gripping and wonderful then imagining what the future could be like. Not to say that the future doesn't interest me, I would guess it interests 99% of the world's population because most people want to know where we are going and what we will have in the future.
But you people to choose and I would have to go with the past in this case, but only because of the time scale you said. If you asked would I rather go back 100 years or forward 100 years (as opposed to 100 million) in that case I would have to pick the future (I only want to go back to the distant past to see dinosaurs )