Chatzos or Tzeis?

The chachamim in the mishna gave a syag that you should not say krias shma lechatchila past chatzos. the chachamim in our gemara give a syag that you must say krias shema as soon as you can. Which one is it?
Most Rishonim answer this by explaining that the syag in our gemara to say krias shma as early as possible only applies if you want to eat or sleep ("ochel kima, ishan kima"). In that case the chachamim say you must not eat or sleep before krias shema. A second syag is that even if you don't need to eat or sleep you still should not wait to say krias shema after chatzos.
The Shulchan Aruch however does not agree with this. In 235, 3 he says - "lchatchila one needs to say krias shema at tzeis, the zman extends until chatzos and bdieved you have until alos". How did the sh"a learn the chachamim's syag about chatzos. they already had a syag to start immediately at tzeis? The mishna berura expains: The first syag to say krias shema at tzeis is to fulfill the concept of zrizin makdimin lemitzvos. The syag of chatzos was to create an issur of waiting past chatzos to prevent people from missing krias shema altogether. If you don't say krias shema immediately you are not a zariz but if you don't say krias shema at chatzos you are over al divrei chachomim.