I don't know about running vs MTB, but it seems to be well documented that MTB is superior to road cycling for bone density, and, in fact, road cycling may be bad for bone density.
Although most research suggest weight bearing as the source of benefit, I also agree with the theory that vibration helps the bone density. I have read an abstract from Europe this year that suggested ultrasound vibration therapy can speed fracture healing. I have found out that some of our local spine surgeons use this technique to speed bone graft healing in back fusions. It has also been theorized that MTBer's stand for pedaling more than roadies and load the joints/bone.
MTB appeals to me more than running as I don't know any folks who have had stress fractures from MTB, but many who have from running. Maybe it's a matter of degree (most of the stress fracture people I know train for marathons). I, for one, feel that 5 to 9 inches of modern, damped suspension goes a long way to give your bones a good deal of weight bearing without a tremendous shock load. Judy, regardless, you should get into it - it's super fun and will increase happiness density if not bone density!
I need to get done with rehab and get back to work so I stop spending so much time on these forums!