A "better" sensor that has more megapixels in this context would be a sensor that has at least the same size pixels as the current one. A 12mp sensor with the same size pixels as the existing sensor would be ~50% larger and probably wouldn't fit in the phone.
Exactly what are you doing that requires you to blow up a 3264x2448 resolution image? Honestly, with the amount of extra noise and terrible low light exposure most 12+ mp cell phone sensors have, you'd probably be better off upscaling the 8mp photo with a good resampling filter anyway. Apple's sensor may have fewer pixels, but it is for all intents and purposes a better sensor than the higher pixel count sensors in other phones on the market.
FFS, I have a 7 year old 7mp Canon point-and-shoot that takes pictures just fine for print. If you're trying to do professional level photo editing with a camera phone photo, you're doing it wrong.