Just to offer a differing opinion... I'd go with King Kong. Kiss Me Kate is lovely, but it's exactly what you'd except it to be, and very predictable. If you see musicals, you've seen it before. A lot.
King Kong may have a weak book and music, but the experience is so different than anything you've experienced in the theatre that to me it's worth seeing. But only if you can sit close - the point of seeing the show is the overwhelmingness of the gorilla, and I think that would get lost from far away or high up. It's genuinely thrilling.
That said, rereading your post, if this is your mom's first show... King Kong is a bit of an anomaly of theatre, while Kiss Me Kate is as traditional as it gets. For a first experience ever, traditional might be the way to go.