Having stayed at both FS Hangzhou and Amanfayun, I much prefer Amanfayun. FS has lovely grounds but the rooms are very corporate in style - more like a city hotel. 80% of guests are Chinese so international guests are in the minority. Also, 99% of staff is Chinese, so no matter how hard FS try to train them, they are not up to the usual FS level, IMHO.
The spa looks incredible but I agree that therapists are very amateurish, whereas at Amanfayun therapists are extremely well trained and very caring.
I know that the lighting can be very dim at Amanfayun, but in every other respect it is a magical and very authentic experience. Getting to Impressions on West Lake is easy from Amanfayun - you don't have to stay at FS to go there.
I also think the food at Amanfayun is far superior to FS, where the all-day-dining breakfast & lunch buffets are a scrum.