Compared to PCs, the PS3 doesn't need as much optimization as trying to get your game to run on a wide array of PCs from low to high end which requires the most testing because of the vast amount of hardware that needs to find compatibility and etc. The PS3 will never change like a PC could in a year, so the PS3 needs the least testing of both supported systems and console betas tend to be "demos" basically because any testing they could do they would do within their own team anyway so public betas on systems tend to be just for hyping it really, so thats why it will happen closer to the final phase of the beta testing just before the retail.
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