For an every day carry: Sig
Because my Sig has been my most reliable gun hands down (Big plus for the Sig. The XDm is as of yet unproven - at least for me)
There is no weight benefit in carrying the XDm (Could have swung the scale in favor of the XDm)
I'd carry at least 1 spare mag either way, and I'd be comfortable with 17 rounds (p220 mag holds 8. Either works)
That's just me.