I know Glocks and M&Ps only need a simple barrel change to switch between .40 s&w and .357 sig. It is probably the same on the XD line. I don't know of a single firearm that needs a different magazine for the .40 and .357.
The .357 sig is essentially just a .40 s&w case necked down to a 9mm projectile.
In my limited experience with the .40 and .357 sig I find the .357 to have slightly sharper recoil. In a full-size firearm it is controllable though.
I don't think there is much advantages to get a .357 sig barrel. Ammo is usually hard(er) to find and is more expensive than .40.
I think you would be better served spending the money on more .40 ammo.