Victoria House (Ambergris Caye) would be my choice for a honeymoon. Less chance there will be noisy children in the pool as Banyan Bay tends to attract families (all 2 bedroom units I believe). If that doesn't bother you, it is a nice property....but for a honeymoon I would stay at Victoria House.
Sorry, haven't stayed at the Inn at Robert's Grove. As beginner divers I think you would like the access to easier dives from Ambergris - also partial to the island as that is where I have my home. cool