There's a lot of confusing info on here, for such a short thread.
Uncle Changs is mega-cheap. Absolutely no frills. Dorm style shared room, with basic food. Friendly.
Scuba Junkies is aimed at the backpacker ('flashpacker') market. They achieve low prices through volume of customers. They have young, energetic, fun staff, meet industry standards and are very safety conscious. The instructors are very experienced and most of the diving staff are dedicated underwater photographers. The new SJ resort on mabul is clean, comfortable and getting good reviews so far. The accomodation at Semporna needed a refurbishment. You get what you pay for. I've heard people criticise SJ for 2 reasons; (1) inter-shop politics/competition and (2) absurd expectations by comparing them with the 5* resorts on Mabul. It's silly to listen to such rumours. Have a look at their Facebook page to get a more realistic feel.
Seaventures. A step up in price. More mid-market. There is a dive site under the rig (16-18m, current, good marine life). Otherwise you will have to go by boat every time for the awesome shore diving around Mabul.
I recommend the convenience of being on Mabul itself...as there are more shore diving options on shallower sites (I can spend literally hours with my camera at 6-8m around the shore). Distance to Sipidan is identical from Mabul or Seaventures. Seaventures has just one dive option (without boat) and that is quite deep if you plan to dive repeatedly. Whilst the Seaventures site is definitely worth a visit (or two), there is a much wider selection of sites around Mabul island itself.