Hi - welcome to the Point Hacks community!
So, according to this, your CBA Diamond card transfers out to the Velocity Frequent Flyer program at 2 CBA Awards points = 1 VFF point. It earns at 3 CBA Awards points per $1.
Therefore, your effective earn rate on the CBA Diamond card is 3/2 = 1.5 VFF points per dollar.
Because Amex MR transfers to velocity at 1:1, for non-supermarket and non-petrol spend (so everything 1:1 and government/ATO/utilities bills at 0.5:1) the CBA Diamond card is better on paper.
However… as with everything here, there’s a bit more to it.
As far as I can tell, CBA Awards only transfers out to Velocity. So you’re effectively locked into using Velocity points. Amex MR has many more partners and transfers to the majority at 1:1 (including Velocity of course). This extra flexibility can be a lifesaver: there are lots of sweet spots using points in Asia Miles or Etihad Guest Miles for instance.
Therefore, the answer is “it depends”. Do you want just Velocity points? Or would you like to diversify a bit with a more flexible points currency up your sleeve?
Also, upgrades aren’t a bad use of points, but it’s better value to try and redeem flights in premium cabins outright with them (at least that’s the general consensus on PH now).
Hope that helps!
EDIT: Just found one of Keith’s guides that values various points currencies, which lets us put some dollar values on the above. He values Velocity points 1.5cpp, Amex MR at 2.0cpp, and CBA Diamond points at 0.75cpp. Therefore, when spending $1 on Amex at 1:1, you “earn” 2c. When spending on CBA card, you “earn” 2.25c. So again, CBA wins on paper, but you can see from Keith’s valuation, that the flexibility adds a lot of extra value per point.