It will remain fixed. When you are offered a loan, the effective interest rates will be quoted as:

BR + x.xx%

BR will fluctuate according to the bank's and the nation's economic performance, the spread will remain the same

Only for illustrative and educational purposes:

Lower BR rates will give lower effective interest rates for you, which in turn is better for you.

This is all so new, so we may not know how the BR would move for each bank, although the mechanisms are understood. but I would probably go for lower spread and higher BR than higher spread and lower BR. The lower BR may actually have higher room of movement compared to higher BR. But this is just my gut feeling and shouldn't be taken in as professional opinion, because frankly, I dont know how much movement there will be between the banks. Will they move differently from each other, similarly, or completely the same? It will probably be the 1st guess (they will move differently) but without any reference or historical data, it is very hard to tell.

But this only goes for offers with similar effective interest rates. If you are offered EIR 4.4% @ BR (3.2%) + 1.2% VS  4.85% @ BR (4.05%) + 0.8%, it shouldn't be difficult to choose.

Understood. It seems that I may have been proven wrong. Do you have a link to that particular iMoney article? Thanks in advance.