Tough call.
The 39 has a dynomite feel in grip width and angle-but it's a single stack.
The Sig has an edge in capacity-and even though it feels good in the hand, the 39 feels better.
As for durability-it's most likely a wash, both have an enviable reputation when it comes to aluminum-framed guns.
Grip and feel goes to the 39.
Capacity goes to the Sig.
Honestly it's hard to go wrong with either.
Now, introduce a S&W 639 into the equation and all bets are off, but that's not the question at hand.
I've owned several Sigs, wouldn't hesitate to bet my life on one.
Don't know if I did any good here or just muddied the water, but that's my $.02.
Best,
Heekma