Hey honestly the Raptor 660 would be a good trail bike and a good sand bike. The Raptor is a little tall so you might have a problem with tipping. The good thing is that it has reverse and that is always a good thing when trail riding. The YFZ 450 is also a good choice. The problem with the YFZ is that when trail riding you might have a hard time since it is so wide, and no reverse. In the sand you will have a blast. The Raptor at first might not have very much power but that is an easy fix with a pipe, jets, filter. Yamaha is out with a Raptor 700 now so the only way you would be able to get a 660 if it was used. The YFZ has a lot of power right out of the box. I guess it all depends on what you do the most. If your mostly a trail rider and some dunes then the Raptor, if mostly sand then the YFZ. If it was me I would get the Raptor 700 they are really cool bikes when you get some modifications put on them. Good Luck.
