Yes, Diamonds is worse than DAD, much worse. For example, serious flaws such as the following set them apart (I will not point out things like Jinx v Tiffany as issues like that in all the films tend to cancel each other out):
1. Its very style jars after the relatively serious OHMSS: Connery is a physical disgrace compared with Laz.
2. Not only was any potential revenge to OHMSS jetisoned, and the very existence of that film denied, a cracking Fleming plot was wasted and totally abused.
3. DAF has Sean Connery and yet wastes any opportunity of using his knackered, ageing looks to advantage. Conversely, in DAD Brozza - for all the faults of his portrayal - actually seems like he is trying to give an interpretation of Fleming's Bond in the situations he finds himself in. The same cannot be said for Connery in DAF.
These are the crucial elements to me that set DAD as a better movie than DAF. DAF's production had all the aces - a Fleming story, Connery: DAD had a lightweight leading man perhaps slightly out of his depth, no original source material, and yet Brozza and the DAD storyline are, IMO, closer to the spirit if Fleming (OK, by way of Benson, perhaps) than anything in DAF.
For me DAD all the way.