How much power are you planning on in the future? A t4 based single can support a LOT of power, no reason for twins unless you're thinking 1500hp+ R3 block build. And even then it's probably going to end up with a head on it that would require the hot side to be rebuilt.

If you're just looking for 500hp now, n9 reason to have all of the complexity associated with twins.

If spool is ultra important, you want to keep the turbine small. But it will limit the peak power. What goes in, must come out!

If you're talking stock block in the future, you'll never need more then a single turbo even if you get a bigger one in the future.