How much difference going from a 100mm to 70mm stem will make depends on a bunch of factors that others have gone over above. I would add that if you plan to go to a shorter stem also consider going to a wider bar. I bet that if you have a 100mm stem you are probably running a bar in the neighborhood of 720mm. Any wider and your bike probably already turns like a bus. I would also add that you have to balance bike fit with how much you want to unweight your front end. Current trail and enduro bikes are lengthening the front end of the bike in order to be able to run a very short stem and wide bars and still allow for a correct seat position. If your bike is not designed like this, then whatever you to your stem will be a compromise. That being said, you should be able to shorten the stem 30mm and be able to compensate on the fit. Again, wider bars will uncramp the cockpit allowing for this change. Depending on your size and the size of your bike you might even be able to go shorter than 70mm. If you are unsure, talk to a shop that knows what they are talking about and they can help you get set up the way you like. Yeah, a 100mm stem will put you way over the front end in most cases making it hard to pull up on the front end as well as putting your weight really far forward when you are going down steeps. Good luck.