Hi Nictee,
I am sure lots of flyertalker will disagree but between the 2 SPG properties, my favorite is really Mystique. The view from the cliffs of Oia is breathtaking and you are only 10 mins walk from the most popular place to admire the sunset, at the tip of the volcano.
The rooms (they are all suites) are great and feel very private because each as its own terrace. Carved in the rock, it is definitely a special place. The restaurant serves very very good food when they just opened (and I tried it). It was at the time not so busy and it felt just perfect.
Vedema on the other hand is on a flater part of the island is a small village. Crossing the village is a challenge if you have rented your own car but the hotel is very nice. Think of old house with quaint charm. The hotel does not however have the breathtaking view of caldera. The restaurant did serve some great food and I had great fun being the only one having a wine tasting with the sommelier. Still, Mystique wins hands down for the sheer location, proximity of the sunset spot in Oia, the rustic and unusual feel of the grotto accomodations.
I would not use 60,000 miles for a redemption on nights. With the SPG bonus, that is 75000 miles and credited to CX would give you a business class companion ticket (80,000) to anywhere in Europe. So in terms of value of the points burned, the biz class tickets seems a better choice.
Congratulations on your forthcoming honeymoon (and I presume marriage wasn't 2 years ago) and have a great time in Santorini.

PS: If staying at Mystique, make sure you book the cruise in charter boat. It is a skipper and only takes a limited number of guests, and it is a great fun experience.