Stress detection simply lets you know when a suspect's about to clam up on you. Some say they can't live without it, many will tell you it's something you can sort of come to develop a kind of sixth sense about on your own.
As for pugilism vs. lockpicking, the biggest problem there is with gaining favor with the Green Hand, the faction that's in charge of pugilism. Since you seem to be above the level you need to be at to acquire the skill, you're more than halfway there already for pugilism.
Pugilism adds 10 points to your tough factor (gear and skills), similar to the way Flirting adds on to your charming factor. Lockpicking allows you to get either the motive or alibi question answered for two suspects per case (you don't have a choice which one the system gives you, it'll pick one). It basically gives you two questions for free.
Which one should you get? That's basically a six-of-one, half-a-dozen-of-the-other kinda question. Both are valuable skills, and whichever one you don't get now, I'd strongly suggest you go for next.
Personally, since you've already got the points picked up with the Green Hand, I'd go for Pugilism, but that's me.
my gut feeling is to go for pugilism, but i don't have enough points yet, whereas i do have enough points for lockpicking. so i'm debating whether to wait or not.
would appreciate any other comments about this.
in terms of stress detection, currently at the level of cases that i'm playing, most suspects will clam up on me on the first or second question, but since i ask them anyway i don't think that stress detection is gonna help me much...