OK, I have both of those guns. Both without an internal lock. They are my favorite carry guns, and they both serve a purpose.
If I'm going lean, then I stick the 642 in my pocket using a Simply Rugged pocket protector and a speed strip carrying 5 extra rounds of Speer Gold Dot Short Barrel .38 +P.
If I've got a good cover-up shirt on, then I carry the 640-1 in a Simply Rugged pancake holster on my belt, along with an orange Tuff Products 5-round speed strip holding Speer Gold Dot Short Barrel .357 Magnum.
The 640 has a slightly larger grip (I'm talking about the original grips that came on the guns) and I can get my pinky on there...it also covers the back strap. The 642 is a tiny boot grip that does not cover the back strap. I find the 642 with .38+P kicks much, much harder than the 640-1 with .357 Magnum. My favorite gun of all time, for carry, looks, size, is the 640-1 and I'll never sell mine. I think it is perfect. However, the 642 also has it's place when I need something in my front pants pocket...I've never tried carrying the 640-1 that way, but I imagine it would be slightly too heavy for pocket carry. They both have their place.
I've been considering getting Crimson Trace grips, but have not gone that route yet. Both guns shoot to point of aim, and I am more accurate with the 640-1. The 642 does fine very close (i.e., 7 yards), but gets less accurate past that.
I didn't address the LCR that you mentioned...I hear that is a great gun with a smooth trigger, but I've got zero experience with it.
http://i304.photobucket.com/albums/nn193/mdritter/SIMPLYRUGGED001.jpg
Good luck, B