MHope, GK has a 96-98% success rate on medium to small tumors. I'm not sure where people assume there are no long term studies, as GK has been around for about 50 years and somewhere around a million people have been treated with GK. Surgery has come a long way also and has become vastly more successful since the use of microscopes back in the 70's. Just as surgery continues to be tweaked by new tools and techniques so does GK. The thinking as to why surgery might be better when younger doesn't really have anything to do with long term studies as much as if you're younger you can recover ( bounce back) from surgery better. Where as if you're older, maybe you won't handle surgery as well. Success rates and regrowth chances are virtually the same from either surgery or radiation. There is a very minimal chance of facial nerve damage with GK though as opposed to micro surgery if that is one of your main concerns. With a tumor the size you have, you have several treatment options. That's good! It is going to come down to your own personal preference. Treatment is a very personal issue to many. Good advice is to seek opinions from very experienced doctors which you seem to be on track already. What ever treatment you decide, experienced teams usually come with better results. Good luck to you.
PS: I was 49 when I had my GK done at UPMC 6 years ago.