If you can get the Horizon Club rooms in Shang, go for the Shang which has a better location. For standard rooms, the Mulia is a bit better I think. Either choices are still excellent no matter.

The Mulia is located in the Senayan area which is south from the CBD. As many people have mentioned choose the hotel based on the location of the offices or destinations you are visiting. Travel time between the 2 hotels can vary between 10-15 minutes with little traffic or 1+ hour with peak hour traffic.