The neighbors across the street went to Couples and had a blast.  They didn't have a bad word for the place.  They say that you get served by waiters right on the beach which Sandals does not do.  I saw the beach from a boat and the place looked very nice, i'm sure it's on par with Sandals.  Grand Lido is a top notch place but more expensive then the other resorts.  They cater to visitors who may want to dress more formally at meals etc.  I'm sure they are great, not my cup of tea but probably worth doing your research.  Beaches is basically a Sandals for people who want to bring kids.  It's a very nice place with all the ammenities that Sandals offers.  If your staying at one or the other you can visit both resorts and do whatever you want except occupy a room.  Several couples went to eat at their nicer restaraunt and were pleased.  We just used them for a bar stop when we walked the beach.