Both lenses are good overall, but I prefer the J3. I like the rendering from J3’s from the 1950’s. My lenses were shimmed. My old 50mm lens comparisons had several J3 lenses in the pack. RFF members misled them to me for the comparisons. The three J3's used in my comparisons did not behave exactly the same way. This has been already discussed here by others. When shimmed appropriately, a J3 is not inferior to a Zeiss 5cm 1.5. Same goes for J8 and Zeiss 5cm/2. The Zeiss lenses are harder to find in ltm, and they cost a lot. I also have a "transition lens" (ZK) that supposedly has Zeiss glass but was put together in FSU. Sometimes it is best not to take such lenses apart to figure out if it is a genuine ZK or a false one. Just shim the lens and use it. I own a 5cm/2 ltm with Zeiss glass and a Leica lens barrel. I was informed that this lens is one out of 200 lenses that was sent to Sweden in return for iron ore. The history part is interesting to me. If you already have a Contax-LTM adapter, get Contax mount Zeiss lenses. They are cheaper. The J3 come in ltm without the need to use such an adapter. This is good.
J-3: from a very old lens comparison project
I used a 1938 Zeiss Sonnar 50/1.5 with a M 4/3 camera here: