I am pretty sure that bashdem has already made up his mind and booked his trip but for future readers of this thread.... ssra30 comes the closest to describing the choice IMHO. During the NE monsoon times from around mid Nov to Feb., Similans would be the better choice. During the other times, I would choose places in East Malaysia (or nearby parts of Indonesia) such as Sipadan/Mabul, Layang-Layang, Sangalaki, Manado, Lembeh. Thailand still has a lot to offer throughout the year but the diving near Borneo is just too good to pass up if you can only choose one.
Regarding diving of the east coast of penisular Malaysia around Perhentian, Redang, Tenggol, Tioman, etc.; let's just say it is a very good weekend getaway for those of us fortunate enough to live here or a good choice (at the moment) for some budget diving. Honestly, it's not in the same league as the Similans or Borneo and Sulawesi.