Actually Ambergris Caye is somewhere between Cozumel and Isla Mujeres. San Pedro is a much, much smaller town than San Miguel. It's like San Miguel was 40 years ago before all the traffic, the Hard Rock type of places and the large hotel development. But the island itself, while not as large as Coz, is considerably larger than isla Mujeres -- it's 25 miles long, half the size of Barbados, and because there really are no roads except in town and along the southern end of the island, it seems a lot bigger.
Seven Seas is not quite as convenient as hotels in San Pedro Town or those at the edge of town, but it's certainly an easily walkable trip into the main part of town.
Rubie's is great for what it is, a simple budget place, but even the new rooms don't compare in size or amenities with the apartment units at Seven Seas or other similar places.
Belize First Magazine http://www.turq.com/belizefirst/