Gen Aslam was deliberately posted as CLS, Kiyani especially kept that post vacant for quiet sometime so that once Gen Aslam completes his tenure as Corps Commander, Kiyani would sideline him from the main stream by sending him to that less important CLS post. I don't know much of the inner things in detail, but major difference between the two is thinking on certain factors which are currently happening. Gen haroon does not wants to follow the steps which Kiyani had been doing since last 3 years, he wanna take a different approach which is not what Kiyani followed after being ordered by the US. Gen Aslam is one of the most competent officers currently serving, he is a thorough gentleman, with good moral / ethical personality. Gen Rashid is somewhat loose on the moral side, just like how Musharaf used to be. I hope you know what i am talking about.
One of the things which may cost Gen Haroon the COAS seat is his not so pro-american approach. That is why he may be made the CJCSC, thus he will get a useless / ceremonial post which will give him no power to change things or head the Pak Army.
No COAS will be made who doesn't mostly tows the line which the USofA wants him to tow.