I prefer flatting the flop, but I want to add that the suits of your jacks matter. If you have K
8
J
J
, then you absolutely want to flat, since if your opponent has the straight, half of the time it is with the J
, making it likely he has a freerolling flush draw to go with it. If you have K
8
J
Jx, then your raise is significantly less likely to get freerolled, since villain will need 2 of his 3 remaining non-jack cards to be clubs. I'm not advocating for a raise in this spot, but it's a lot less risky than if you don't have the J
.