hb04103 said:
Version II is the same optically as version I, but it adds weather seal.
In addition to weather seal, version II of the 1.4X also added an improved black anti-reflective coating to the inside of the converter barrel, which could have some positive effect on image quality in adverse lighting conditions, such as shooting into the sun.
My original version 1.4X is my oldest piece of Canon equipment at almost 20 years old. I would not let go of it, but my relatively infrequent use of it does not really justify an upgrade.