I lived in Grenada for 3 1/2 years! It's an awesome island; great people, great food, awesome beaches! HOWEVER it's very quiet in comparison to Antigua.
In a nutshell, both places are very similar in terms of sun sea sand however if you want a quiet, romantic, personal time then Grenada is for you!
Depending on your budjet, I can recommend places to stay! Best of luck deciding and Congratulations on your anniversary!