sorry I’m a bit late on this one…
My humble opinion as far as sleeping bag rating is to go with a “extreme temp. rating” of –25C + silk liner…or – 20C + silk liner + your soft-shell
. For info I had a bag with “extreme temp. rating” of – 27C with out any liner, and slept with my soft-shell when I was in Berlin. 
Down jacket ? I was among those lucky guys who summited in one of those rare days when temp @ the summit is positive (in C)
, had just 2 T-shirts and a micro-polar 
… yet, I vote for the down jacket !
1- The “Marca expedition” of the Spanish sports celebrities reached the top just 2 days before me. Back in Berlin Juanito Oiarzabal who led the expedition (with Juan Vallejo) told me they had about –20C the whole day and snow deep to the knees in the Canaleta…:shock:
2- If not during the climb, you’ll definitely appreciate the comfort of your down jacket (or its synthetic equivalent) @ the camp in Nido (5300m) or Berlin (5900m) while being static and acclimating. You can easily face –10C in the day at those altitudes. Just imagine exiting your sleeping bag in the night and hitting –25C to go pee (better have a pee-bottle though
) or to vomit (it can happen to anyone 
) !!
For info I measured a mini of –11C inside my tent in Nido and a mini of –11 in the shelter in Berlin.
Que te vaya bien companero !
