Put a big asterisk next to that statement. It all depends on what you mean by 'mobility'. If you are speaking of tactical mobility, maneuvering under fire, then tracked vehicles are superior due to their lower ground pressure. If you are talking about operational mobility, then a wheeled APC like the stryker will have better range, speed & sustainability. Road marches would be much easier with the Stryker. Finally, strategic mobility would go to those Stryker variants which actually can be loaded into a C-130. AFAIK, some, like the assault gun variant, can not.
I have never ridden in a Stryker but spent a great deal of time in a Bradley. IGoing cross country (even in the desert) in a Bradley is like being a pebble in a tin can that is being shaken. We were exhausted and/or bruised on every sortie. And that was before we dismounted. From what I hear, the Stryker is a lot easier to ride in and it is considerably quieter than any tracked vehicle. It's quietness is a great asset in a raid.