I agree with Drew.
I used to work in Mabul and the diving out there is awesome. As Drew said, make sure you hit it at the right time of the year.
The company I worked for offered diving in Mabul, Kapalai, Sipadan, + (Mantabuan, Sibuan, Semporna Strait and Mataking if you stay in Semporna).
These are all islands with several dive-sites on each island (no way you will see all of those sites in 14 days and some you might wanna do 3-4 times to get the right shot of those flamboyant cuttlefish eegs or whatever crazy-cool thing you will find.
If you are going to go Sipadan/Mabul, then the one thing I would strongly recommend is to put on a 60mm macro and jump into the dirty (filthy) waters of the Semporna strait. The so called pontoon (fish-farm) was my favorite site, especially good for night-dives! Loads of wonderful and unusual critters living in the muck.
The only disadvantages are that most people (believe it or not) does not want to dive the strait (did I say that its pretty darned dirty?), so it might be hard for you to get enough people together for it another disadvantage is that that you will have to spend a night or two staying in Semporna before/after you go to Mabul.
Semporna is, ehm, nasty 
Enjoy!
-Morten