I have seen many positive posts about the BlockRockit on Yahoo longarming groups. No negatives that I can remember. I would contact KathyQuilts about the frame options. I have seen some negatives about Grace frames, that at least some of them have weak rails that tend to bow in the middle under pressure.
I couldn't find much information about the King Quilter. The only source I found is sewingmachinesplus, which has some Youtube videos on it. Could not find any actual user reviews or comments about the machine other than the two reviews on the sewingmachinesplus website. It could be wonderful, or it could be one of those machines that has lots of options but never works quite right.