My husband and I chose between FS and the Amans for a one week stay in Jimbaran Bay followed by another week in Ubud. We decided on FS for both because, although both were expensive, FS seemed slightly more reasonable. We had a villa looking out on the sea at Jimbaran and one of the newer villas up on the ridge rather than down by the river at Sayan. I chose the ridge villa over the riverside ones after I rang the hotel and one of the staff told me that the river villas had lots of mosquitoes at that time. So, since he was so nice, I asked him to please put us in the nicest ridge villa available since we were staying a total of two weeks in FS Bali hotels.
We were very pleased with the accomodations provided at both. I loved the Jimbaran Bay villa and it had a beautiful view. Yes, you can see the airplanes but I don't recall it ever bothering me. The interiors were nice and very local.
Our Sayan villa was reportedly the one that the owners use when they are there. It is one of the newer/ newest villas so the interiors are modern and lovely. I had a chance to look at several of the river villas and they indeed looked old and worn-out compared to our ridge villa. Our ridge villa was also much quieter. The river is the one of the attractions at Sayan, but if you are concerned about shabby-ish interiors, I suggest asking the hotel to give you their newest villas.
Also we ate at the Amans and the food was the same quality as at FS : good and enjoyable enough, but not a Michelin 3-star meal. In fact, FS had more variety. I would stay there again.