Trust me on this one, SD, if you are never going to carry it in the pocket, go with the 640. On a vest holster, or belt, the weight doesn't make much difference, and the 640 is a hell of a lot easier to shoot than the titanium, and you won't ever wear it out.
I carry a titanium, and they are a bitch to shoot. They're accurate and fast, but I can usually only fire 20 rounds in practice before my hands bloody. The 640 took me thru a 3 day class with no problems. They'll get you thru a gunfight just fine, but alot of practice is NO fun at all.
If'n you're going to carry on a belt holster off duty, go with your duty gun (or a compact version), 'cause you gotta dress to conceal it anyway, and if you can conceal a .38 revolver, with the proper holster, you should be able to conceal the duty piece.
Moon