Personally, even if it does happen to be that sanguine pawns, when they do break, are more violent or have less desirable types of breaks by comparison to iron-willed pawns I'd still prefer the former over the latter. But that is largely due to the fact that my pawns don't tend to break even without those two traits, when they do they're 'soft' breaks. I could probably count on one hand how many games I've had where a pawn has had a severe break down.
So, sanguine only helps as it improves my pawn's mood additionally on top of what they have, while iron-willed just prevents them from breaking (which they typically don't, even without it). Admittedly, I haven't done a lot of testing to give a highly educated opinion, because I don't really see either of the two as my top desirable traits. Definitely better then some, but it doesn't really make me think differently, like trigger-happy does.
So, sanguine only helps as it improves my pawn's mood additionally on top of what they have, while iron-willed just prevents them from breaking (which they typically don't, even without it). Admittedly, I haven't done a lot of testing to give a highly educated opinion, because I don't really see either of the two as my top desirable traits. Definitely better then some, but it doesn't really make me think differently, like trigger-happy does.