Here's my take on the issue. I like both engines but one more than the other. My pick is the 103 for several reasons. There is less valvetrain mass and less parts. The 103s single row chain is still just as durable because it's 'single cam' load is less. There are a lot more cam advancer issues with 104s than distributor ignition problems on the 103s. I've seen a few 104s with pronounced piston slap around the 100k mark I've only ever seen one 103 with bottom end problems. (that wasn't the result of being a moron) The #1 exhaust valve broke (blame the valvetrain for the failure) and it took the piston off the rod and put it in the oil pan. Meanwhile this car drives to the shop with a loud knock. Upon disassembly I found the rod whole with the pin still in tact. It had beat a 1 inch groove into either side of the bore. I remember being impressed

FWIW Although I'm biased towards the 103 my boss at work often jokingly refers to me as "104" given the sheer volome of those that I work on.

I guess that my biggest bias to the 103 has to do with the pair of turbo manifolds that I have which are 103 specific