Best is slightly subjective on my part. If that's the word I want. Since I had the headframe on three times, two being very painful, I much prefered the tight mesh face mask of the CK. Time frame for me was different too. Ie: from 7am to 6pm for SRS. In at 10 out by noon for the CK.
It is my understanding ( and I did post it somewhere on one forum...oh it was here I believe) the software for the melding of CT, MRI to map the delivery of beams is most up to date with the CK.
I think Rose makes a good point tho' for experience and training.
I think they are pretty darn close to being 'as good as' eachother.
The only problems I have noticed/read are when the GK is used for other types of brain tumors or abnormalites. No one has reported any long term/short term side effects when brain mets are treated, to my knowledge.
Hope this helps a little bit.