I'd much rather run outside than inside on a stupid *%*%*&*& treadmill, but even on the grassy areas it's not so great for your shins and knee joint .... a treadmill isn't much better, but those that can be adjusted for weight do help somewhat ....

personally, I do cardio within my workout (by design, with the reps and rest time) and then use the eliptical afterwards .... feels kinda dorky but it doesn't kill your joints and bones like normal running does, and is just as effective if you pay attention to you HR