I'd get the .357 Sig, personally, it seems to be less sensitive to shorter barrels than the .40 S&W. Remember, the 229's barrel doesn't quite make 4", which technically puts it into the "short barrel" category; and the .40 seems to lose a lot of OOMPH when you launch it through a short tube.
Of course, the .357 has lots of flash and blast in 4+" barrels, so the fireball should be pretty spectacular out of the P229.
I personally don't consider the 229 platform suitable for a "backup" gun - unless you're talking about a concealable off-duty gun that'll be your defacto primary while you carry it. Reason I say this is the 229 in .40/.357 is VERY heavy and wide; it's not really suitable for ankle carry, and it will add A LOT of weight to the belt if you try to carry it off-side from your primary. Just for grins, what is your primary anyway?
If you're dead set on Sig, I think the P239 family makes a better backup. Still to heavy in .40/.357 for the ankle, but it's flat enough to conceal fairly well in a "vest" holster on the straps of your body armor.
Regards,
Kevin