If Captivate worked on everything I would use it over Flash in a heartbeat. That being said, it does not, so Flash is the better option of the two.
I agree with what you said about the attack IV, attack isn't a huge issue. I hadn't looked into speed too heavily, I merely thought that the extra speed could be handy for getting in an Accupressure before your opponent gets in a hit, possibly saving some HP.
Also, I did correct your post by hand, I'm an English major, so it irks me. I don't like making a huge deal about it, I just do it while I'm thinking over my post.
I'm going with Impish right now, purely because it's a pain to find a female Careful Skorupi when all you're doing is breeding for it (1/50 eggs will be female/careful.) I have two Skorupis, male and female with perfect HP and defenses, so breeding for better speed and attack wouldn't be too hard, I'm just not certain if it's worth the effort if I may need to switch to Careful later. I have noticed that I can't stall out certain pokes locked into Surf or Ice Beam or the likes, so a point for Careful, I suppose. On the other hand, I do enjoy seeing opposing Garchomps do laughably poor damage to me.