doesn't hurt to have more than enough,especially if you figure the 15a is minimul,then you end upupgrading your Torus later.of course you could use the 15 for components and sub.
A few extra dinaro,you could have the 20a.
I found with my amps(5)the 15a didn't seem to like all those amps on it's back,as soon as I installed a20a,the skies opened up,I couldn't believe it.
As James would say:having gobs of reserve power makes a HUGE difference,
I can testify to that,if you can afford a bit more(especially if you plan on upgrading),
do yourself and the music a favour,you'll be happy you did.
I have seen people on Audiogon sell their Torus(S) to buy a BIGGER Torus,as they realized that with
their power requirements that they needed a bigger one.I'm one of them.
And it makes a BIG difference

.