I'll second a vote on the 638. Give you the option of a single action pull if needed / wanted. I've had my 638 for a fairly short time, but really love it. It has been my primary carry gun since I picked it up. It's so light and easy to carry, you barely know it's there.
Thus far, I've worn it in an IWB holster, carried it in my front pants pocket and carried it in a jacket pocket.
BTW....the Smith's web site gives identical dimensions for each.