Big question is if you are going to reload.
If so, then you can use lighter Gr bullets in the 40 to get around the speeds of the heaver loaded 357 sigs, and when it comes to buying case, the 40 are just way cheaper and easier to find than the 357sigs.
Bottom line if we are talking factory loads, the 357 sig is just a 40 case necked down to 9mm (read 40 case volume over a 9mm case, and with the lighter bullet, less felt recoil). So on that, if you want a fast 9mm load, it's the 357 sig. If you want a fast 40 load, then buy a 10mm.
And as for the 40 being a tack driver, I have more than a few race guns that can prove that point, and do it so smoothly that you would think that you had a BB gun in your hands.