The head is the biggest difference,the ports on the HO are raised 5/8" allowing a more direct path and more flow. That is why the Renix and HO manifolds don't interchange,the ports won't match up.
The blocks are basically the same but IIRC the Renix cam is advanced 7 degrees over the HO. Some also claim the Renix blocks have higher nickel content and are stronger but I don't know if its true.
The computers are different,most of the sensors will interchange but the TPS signal is reversed. Also the CPS reads different so the flywheels have to be matched to the system used.
You can actually put the entire HO top end and sensors on the renix and keep the renix computer you just have to convert the TPS to sweep the other way and swap the throttle cable.