I don't own either, but have had a good bit of trigger time on both, because I have friends that have them. I prefer the SW. Like you said, the Sig (the ones I've seen) and pretty finicky with ammo.
With the two Smiths that I've had trigger time with, they ran without a hitch. So far, I've run Federal bulk, CCI blazer, CCI Standard Velocity, CCI Mini Mags, Federal Auto Match and a couple of others. I can't comment on 20 yard groups because we didn't shoot for groups. I will say that sighting in at seven yards produced nice tight holes and at 25 yards hitting shot shell hulls, golf balls and clay pigeons wasn't difficult at all. What I didn't like about the SW was that they only got one mag with each of the pistols and extras are around $30, each.
Honestly, I think I'll be picking one up when I finish paying for a project that I'm working on. Of course, you should also consider that the Sigs are pretty reliable with ammo that they like (although the choices may be more limited) and the grip frame is smaller on the Sig (I don't know how old your son is).