From both of those descriptions it sounds as if there was some attempt to conceal the bodies. They weren't just dumped in plain site. Maybe Sarah was also placed semi-close to a road and quickly covered up, she just hasn't been found yet?
I was thinking about what would cause the killer to change his disposal method, but maybe he didn't change it.
The article posted above tells of a call the Spiers' received that gives a hint of where Sarah's body is located. It is the one call that her father believes is legit. I wonder what makes him think the caller had real knowledge (as opposed to the many fake contacts) and is there any other evidence to support Sarah's body being in this location?
I struggle to believe SS wasn't well hidden. I think he changed his disposal method.
I think he rationalised that what he did with SS was not the best option for one, or a number of following reasons;
1. Time. It took him too long to dispose of SS. He needed to do it quicker to reduce risk of detection
2. Time 2. He realised he wanted to spend more time with the bodies and to do this he had to a) abduct the girls earlier in the night, and b) dispose of them in less time
3. Distance. He may have taken SS a lot further out to dispose and felt this increased the risk of a random police stop or his vehicle being spotted
4. Distance 2. Also related to time. Driving further out may have taken too much time.
5. Publicity. He may have felt he needed the bodies to be found to enhance his experience.
6. Captivity time, or time with body. He may have kept SS overnight and dumped her the next day and for whatever reason decided he had to do the whole op in 1 night, meaning less time to dispose.
7. He may have buried her in his back yard and then seen a media article about another crime where a body was found on the killer's premises years later, and even though there was no DNA the killers fate was sealed. This may have prompted him to dump the bodies where it couldn't be connected to him geographically.
I think his disposal method with SS was different and he decided he needed to change. SS was abducted approximately 2am and at that time of year sunrise was 5am. 3 hours. Not a lot of time considering he probably spent a lot of time for disposal.