He has outstanding "wiggle", which is what I (and others) call the combination of tackle breaking ability and balance. He is very good at keeping his feet after being hit and falling forward. His vision and cutback ability is good, but not great, IMO. With the line that KC had (has?) he didn't need that. In comparison, LT2 does not have as great of wiggle, but he has fantastic cutback and vision. Superior to LJ.
I think you see this most clearly comparing them at the different levels of the defense. At the first level (DLine), LJ is not used to seeing much resistance because of the outstanding holes his OLine provides. So just getting through them quickly with a head of steam is enough. LT2 needs to use his vision more to find the hole and slide through it.
At the second level (LBs) is where the biggest difference is, IMO. LJ can use his excellent wiggle to get through glancing blows of LBs, but a LB with a read bead on him will stop him because his vision/cutback ability is not usually enough to fake the LB out. LT2, on the other hand, has outstanding cutback moves and can get through that second level of the defense easier because of them.
At the third level (DBs) LJ's wiggle really helps him shrug off the tackle attempts of the smaller DBs. LT2 doesn't have that as well, but his vision can still help him fake out the DBs. I also think that LT2 has better breakaway speed.
In a nutshell, when LJ gets through the first level of a defense in a hurry due to outstanding running lanes, he's hard to stop. But if he is slowed up some there, I think he's easier to stop at the second level than LT2, who can really get by the 2nd level and 3rd level with his vision and moves. I think that due to these differences, LT2 is the better RB with a poor to average OLine. I would expect that LJ will not perform as well this year if the OLine does indeed falter, but he still should be a great RB.
Just to add an important point: I think that LJ is much more dependant upon the play of his OLine than LT2. So I think there is more variance with LJ and that makes him more risky.