I don't really have specific answers to your questions, but here's one point I would make: The lighter 40sw rounds (135-140gr) made by Black Hills, CorBon, etc will provide similar ballistic performance to a 357sig....which might save you the cost of a barrel and potentially some springs.
I have a 357sig (Sig p226) and it's a fun little thumper, but it costs an arm and a leg to feed. Range ammo is almost as expensive as defense ammo. If you've got a solid setup it probably is a flatter shooting, more accurate round than the 40sw. Of course I'm not nearly capable of demonstrating that.