I've been to both resorts and for an adult couple traveling without kids, the Royal would be my choice hands down, even for one less day. I guess I'm a quality over quantity person but of course not everyone feels the same way. The food quality alone would be one reason, but the adults only atmosphere, nicer beach area, upscale rooms are all reasons for my choice of the Royal. The Gran was "OK" but just not in the same league as the Royal.
So...enjoy the Royal!