Looking at the 2 options provided by you, I can say IIM Amritsar is definitely a better choice. I have seen many people regret going for CGL. 

Excise Inspector is the worst post of CGL and many of them retire as superintendents themselves. The transfer is banned in CBIC so no matter which remote area you have been posted to, you have to stay there for 15-20 years until you become Assistant Commissioner which is rare.

You can change your life in two ways 

• Crack UPSC
• Or leave the post and prepare for CAT

You can become a CEO through the second option. I have many acquaintances who left the CGL and went for CAT and today are entrepreneurs or CEOs. Surely the job at CGL provides security but the job itself is not worth it.