A lot would depend on the time frame and the setting (CONUS, overseas, war or peacetime, etc.). During 'Nam and for several years following (frequent forays into "Banana Republic" countries in Central and South America for various agencies), I carried, in addition to issue weapons (in a "declared" war zone) a 4" Model 15 in .38 Spl in a shoulder rig beneath my fatigue blouse - also carried a 6.5" Mod 29 .44 Mag the same way, but it was much more difficult to conceal, and ammo availability was zip outside CONUS. When stateside, I often carried a 4" Python. Later on, I was in a reserve unit (not SF any more, branch transferred to Medical Service Corps in the late '70s) which was a rapid deployment outfit. When President Carter was conducting the "peace talks" between the Egyptians, Palestinians and Israelis (the Camp David Accords) in, IIRC, the late '70s or early '80s, I was the XO of of the unit, and we were getting ALNOTs (Alert Notices) almost every other day, looked like we were going to the desert (it was a much hairier time than was ever reported to or by the press, and he had put us squarely in the middle of it). I picked up a 2.5" Model 66 to carry concealed, should we be deployed (again, in a shoulder rig beneath my fatigue blouse). Fortunately, we were not deployed, and I didn't have to use it in the battlefield, though I know from experience with my 15 it would have done well if the need arose. The choice of .357 is a good idea, as .38 Spl ammo is (or at least used to be) available through Army supply channels (.357 Mag, of course, is not), so ammo re-supply should not be a major concern, at least theoretically, especially if he has a "Get out of jail free" card (do they even have them anymore?). Anyway, of the two mentioned, I would gravitate toward the 686, as it is not that much more difficult to conceal than a 3" Model 60 (the 3" barrel decreases its potential for pocket carry, so either would require a good holster for concealment). Grip selection would be critical, though, with a good set of aftermarket "combat" style wood grips replacing the stock S&W target stocks making it much easier to conceal. The size, capacity, accuracy, reliability and overall ruggedness of the 686 lend themselves to the mindset of a troop experienced in combat, especially anybody with a "black ops" background. Good luck with your project.
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). It's the weapon I've been carrying since 1990 for self protection and carried on duty since 1978 before the military in their not so great wisdom went with the M9, which was a bad move IMO. Eight shots versus 6, more accurate thanks to the longer sight radius and way faster to reload. 
