Last year, IMO: the South Andaman divesites are at least comparable to Similans with regards to amount and variety of marine life. Hin Daeng, Hin Muang and Koh Haa have been really excellent, and offer some excellent diving. Visibility though is unpredictable there, except at Koh Haa where it tends to be good. Also the other southern sites like the best sites in the Phi Phi are and the Shark Point area are really nice, King Cruiser wreck at the moment even better and excellent.
At the Similans there are 5 diving areas: Similans itself, maximum on par with some of the southern sites but not as good as the best sites there, Koh Bon, Koh Tachai and Richelieu Rock, which were really nice diving last season, and Surin Islands, where there is not much diving done and conditions are more unpredictable.
So I'd recommend either a minimum 4 days trip to the northern sites with the focus on Bon, Tachai and Richelieu, a 3-day trip exclusively to Bon, Tachai and Richelieu, a southern trip and/or a combination of those.
Now if you mean 'Andaman islands' as you mention, so India, the story would be different of course.