Sleeping bag weight

[buglebrush]

Help me choose!!

Nemo sonic 0°
Big Agnes Blackburn 0°
Mountain hardware phantom 15°

For backpacking and I'm a cold sleeper. Running an xtherm

What time of year?  My wife is a cold sleeper, and my 15 is plenty for her until October.  I don't switch from my 30 to my 15 until october.  Wouldn't ever want a 0 in summer and september.