I haven't got vast experience, so i was waiting for someone more qualified to answer, but here are my thoughts-
I dived at Aliwal Shoals and Sodwana Bay in April. I couldn't dive Protea Banks as you need AOW for those dive sites- they are deep and have quite challenging currents that can whisk you off into blue water. I did my AOW in Sodwana Bay, but after I was in the Protea Banks area.

In April, water temperatures in Aliwal Shoals were about 23, in Sodwana 26-27. I only did 2 dives around Aliwal Shoals and found it had pretty poor vis, although we did see some sharks (ragged toothed sharks). I gather the Aliwal Shoals/Protea Banks area is particularly good for sharks. I dived with the Aliwal Dive Centre- http://www.aliwalshoal.co.za/ who seemed well run, professional- I had no problems with them. In August, I imagine the water temp will be lower that the 23 in APril. However, at that time of year you have a chance of catching the sardine run (I believe)- you should look into it, and if it is the right time then I would definitely try to catch it as it's supposed to be quite a spectacular phenomenen.

I generally preferred the diving at Sodwana and I did quite a bit there as I did my AOW (with Coral Divers) and a few other dives. I'd definitely recommend Coral Divers as an operator, and they have accommodation on site to suit various budgets. The diving was warmer, much better vis and there was plenty to see. I didn't see any smaller sharks, but they have lots of huge potato cods and groupers, and lots of interesting smaller things- ghost pipefish and frog fish, for example. I did also see one whale shark which was fantastic.

For all the diving in South Africa, be prepared for diving from RIBs launched from the beach- you may have long and bumpy rides to the dive sites, and getting in and out of the boats can be physically strenuous.