I'm sure both of the 24 valve heads out flow the 12 valve head.
But by the same token, we shouldn't discount the benefits of the other improvements made to the M104. For example, on the 340 V-8 Dart in 1968 a windage tray was proven to be worth 15 horsepower on said engine at 5,000 RPM. How much more would it be worth on a longer 6 cylinder crankshaft at 6,000 RPM? I'm sure it was more than 15 horsepower but for this discussion, we'll be conservative and stay with 15. So there's 15 horses just from the windage tray that the M104 had and the M103 didn't have.
Also the 1 point rise in compression ratio is worth a 4% gain (an accepted rule of thumb) in power. We'll only allow 3% to be extra fair which equals another 5 horses for a very conservative total of 20. That's 20 of the 50 that the M104 had over the M103. Food for thought.
Guy's, here's a site to make your heads explode:
http://users.erols.com/srweiss/tablehdc.htm
Regards, Eric