The magnum framed 649 gives you a 1/4" longer barrel, which gives you an additional 1/4" of ejector travel. This almost fully extracts a 38 Special case, which is why I prefer the 357 version. (Same holds true for the 640s.)
I have shot one (1) round of 357 out of one of these guns - with the biggest, softest Pachmayr grip available plus a shooting glove. It still hurts. So I can say I've done it, and also that I'll never do it again. The +P 38 FBI load is plenty for me out of one of these things.