Re: the difference between floor GHRs and the "machine" GHR.
Floor GHRs are much cheaper and more accessible to a larger population. That being said, I don't think flexing/extending the knee while the patella is being pressured against the ground (whether or not on a pad) is a good idea. Sounds like a perfect recipe for chondromalacia patella to me.
If I was going to add GHRs to my routine, I'd only go with the "proper" machine set-up, with the pad sitting on the front of the thighs (no pressure on the patella at all).