SGSL has a nice beach but IMO it's not the quintessential white sand, turquiose water you are looking for. Having said that, I wouldn't hesitate going back to SGSL. We really liked it there and we are beach people always looking for that perfect beach combination you described. I don't think the Pink Gin beach in Grenada will be either, but I'm relaly looking forward to going there in April. In Grenada the beach closest to what you are describing is probably the Grand Anse beach. We stayed at Spice Island many years ago and have wanted to back ever since. We are booked at SGG in April 2014.
If you really want the perfect beach I'd suggest you consider Beaches T&C. I know there are children there but we loved the beach so much we could over look the kids. Whitehouse's beach in Jamaica is another one you might want to consider. However in Jamaica I perfer the beach in Negril. My favorite beach in Negril is at Couples Swept Away. I'm only making this comment because you are looking for the quintessential beach and I'm not trying to pull you away from a Sandals property.
I haven't been to SEB but from what I've heard from friends the beach is perfect. Its on my list of places to visit soon. It might be what you are looking for.
If you can be happy with a really nice beach and a great resort, SGSL and SGG are worth serious consideration. Grenada and St Lucia are very nice islands that offer lots of unique off property experiences. Both islands are very romantic and perfect for a honeymoon.
Have fun making your decision.