Secret Chimp - The reason why manifold or ported works better comes from the combustion characteristics at idle. Here's where the generalizations can't cover all the combinations.

Burn characteristics are due to compression, chamber, valve timing (cam) and load. Hot rodded engines usually prefers ported because the valve timing favors higher rpm over low rpm. Richer idle mix offsets the exhaust dilution although many of us chased our tails for years trying (mistakenly) to lean out idle on cammed engines.

Closer to stock, the timing needs are as varied as the combinations created. There were also arrangements that mixed manifold and ported. It is not a difference between brands, nor entirely about emmissions. I used to think that too but the facts are otherwise. See the responses to my question on this page for more about OEM solutions. (especially post #19,20)

Also, in above linked thread is a really good and detailed explanation of the reasons for choosing ported vs. non-ported advance. Specificly go back to post 10 second paragraph & more in post 13.