We've used the Mathnasium and are very happy with them. They tested my son and discussed his placement with me. They were very flexible about how to approach the best way to help him. They worked with my son at his own pace, and they test occasionally to check progress. They DON'T assign homework (unlike Kumon), which was a big plus for us. The guy who runs the place told me that their philosophy is that if they have to give homework, they aren't doing their job properly.
Kumon: we looked at them. VERY inflexible. Kumon is all about drills, timed tests, accuracy, and more drills. Accuracy is obviously important, but IMO, the Kumon approach might not be great for HG+ kids.
