One, I would probably go w/ the P229 as it was built from the ground up as a .40, & I prefer the size & feel of the Sig. Both are good guns, and as BobCA said the shear mass of the Beretta Elite really helps tame the recoil of the .40. So, if recoil is a major concern check out the Beretta, although the P229 handles it fairly well also. As far as converting to 357 SIG, you can do the same w/ the Beretta using an aftermarket bbl. I'd suggest renting & shooting both if possible.
Ed Ely, I think you have it backwards, many more LEA carry the .40 as opposed to the 357 SIG. There have been some agencies to adopt the 357 SIG however, the .40 is in use w/ many more agencies than the 357 SIG, including the FBI, DEA, & Border Patrol.