I understand that the diving in Lahmi (have only been to Shagra and Nakari camps) has a different layout. There are no house reef just off the beach and all diving is from zodiacs. This might swing your choice in one or the other direction. Personally I give a slight edge to Shagra because I find its house reef more varied and better and the proximity to Elphinstone, giving access to a real off shore fortress only 20-25 minutes away.
What it comes down to is that one has to visit all three for one week each