I'll second tnoutdoors9's videos on youtube because he's done alot of testing with both the .38 and .357 defensive loads, including several out of short barreled revolvers. Brassfetcher.com also has some ballistics gel test with both calibers, just be sure to check the barrel lengths in the test. Compared to a service length barrel (say 4 to 6 inches), a snub nose produces much less velocity and bullets may not expand (and non-expanding bullets go through lots of stuff before stopping).
Loads that seem to get good penetration and expansion in both snub nose and full-size guns are the +P all lead 158 hollowpoint semiwadcutter, the 125 +P Golden Sabre and the 135 +P Short Barrel Gold Dot. All these loads will work in both the 642 and 686. After you get comfortable with hot .38 specials in the 686, you might want to try the 125 gr jacketed hollowpoints in the .357 mag. Most consider them as some of best defensive loads ever produced. They will be loud and have some recoil, so double up on your hearing protection and go back to .38s if you start flinching.