Whoops, I realize I didn't complete my argument.
SIA is most likely choosing Rolls for the 777 and Pratts for the 744 because they believe either of these things:
1) That is the best engine available for the airplane
2) They got a much better deal on one engine type than the other, so much to negate the savings effect of an all PW or all RR fleet.
These answer is simple, but they makes the most sense.
I should mention that Air Canada also has GE products indirectly with the many CFM56 used in A319, 320, 340 as well so they are going with all three manufacturers - as are other airlines.