The C64U is capable of 8 SIDs. Why 2 when can use 8?
I kinda had the impression that old Ultimates support just 2 SIDs and thus the base for 2 SIDs would be bigger than C64U 8 SIDs, but now I'm questioning that idea... But true, if C64U and 8 SIDs is the new baseline then go for 8 SIDs!