"Two is one, and one is none." I think it's a good idea to have more than one BUG/EDC available. If one of them breaks or needs maintenance/servicing, then you've got the spare handy. If the 638 is to your liking and it's a good deal, buy it. You can probably use the same holster and definitely the same speed strips and ammo. The only thing better would be a spare 642.

The last alloy framed gun I looked at was a Scandium framed .357. It was a consignment piece but the frame was cracked under the barrel stub. Be sure to look over that 638 carefully.