ASO in Railways VS ITI in CBDT :-

1. POWER - Although after faceless assessment, power of ITI, ITO and other designations have taken a significant dip yet I would put ITI above ASO in terms of power because of some particular sections in CBDT like WING etc.
2. SALARY - Both are 4600GP post so you will get same salary. Some perks may be different like you get 18,000 Bonus in ASO. On the other hand, you get 30 litres of petrol in ITI when you have field job.
3. LIFESTYLE - You can have same lifestyle in both departments.
4. WORKING ENVIRONMENT (WORKLOAD) - Both departments can be hectic at times if you are placed in some heavy work section and both departments can have a very relaxed environment if you are placed in some less work section.
5. NATURE OF JOB - It would be always same in ASO as it is Completely desk job but you can get variety in ITI as it has both desk job and field job which is termed as Assessment and Non-Assessment in CBDT department.
6. POSTING - ITI has posting in all parts of India whereas in ASO, you get posted in Rail Bhawan in Delhi most of the times. There is possibility that you are transferred in some other state in ASO but most people get posting in Delhi only.
7. TRANSFERS- ICT is banned for the time being in ITI so don’t expect transfers. In ASO, you are posted in Delhi only so no choice there. Although you can get transfer in your home state if you want in ASO as well (PLEASE CROSS-CHECK THIS POINT AS I AM NOT 100% SURE ABOUT THIS ONE)
8. PROMOTIONS - Both departments have decent transfers so you will reach upto level 12 or 13 easily in both departments. Above that level depends on your age of joining, your category, department policy at that time and many other parameters.
9. INFRASTRUCTURE - It is very good in Rail Bhawan. You can get anything from 10/10 to 4/10 in ITI which depends on your posting location.
10. UPSC Preparation - AIR 1 in UPSC 2019 was working as ITI before clearing the exam so no surprises there that in ITI, you can prepare for UPSC. As an ASO also, you can easily prepare for UPSC.
11. PROS of ASO in Railways -
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    1. Railway pass is the biggest benefit here.
    2. Ministry job so relax life. which means you will get enough time for trying other avenues of life.
    3. You can be posted in Delhi throughout your life unless there are some department policy changes.
    4. You are sitting in Ministry so you can make some high level connections from ministers and other politicians.
13. CONS of ASO in Railways -
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    1. Monotonous job. You will have to do similar work throughout your life.
    2. Promotions are stopped right now due to pending court case.
15. PROS of ITI in CBDT -
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    1. BHAUKAL in people’s mind is the biggest benefit here. This department is considered one of the most prestigious departments among all government offices in the minds of general public.
    2. Desk job plus field job combination. You have choice to work as per your preference.
    3. Promotions are good in CBDT as it is revenue generation department so new vacancies keep on creating as tax payers keep on increasing.
    4. You can be posted in your hometown which is the preference of more than 99% of aspirants.
    5. Power in CBDT will be much more once you reach ACIT and further designations.
17. CONS of ITI in CBDT -
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    1. Transfers are banned. If you get posting in another state which is far away from yours then you MAY regret your decision throughout your life.
    2. It can be hectic at times especially in the month when ITR filling is very much.
    3. Faceless Assessment has reduced power of ITI and ITO to great extent. Now you need to work in some particular sections to feel the power.
19. VERDICT - If you get posting in your hometown in ITI then ITI might give you more job satisfaction than ASO BUT if you are posted miles away from home in ITI then ASO would be preferred. DISCLAIMER - It is my personal opinion. You might differ from my thinking.

I have tried my best to compare both posts without prejudice. I hope it helps you in deciding your post preference for SSC CGL. Although it is advised to connect with some ITIs and ASOs before deciding your post preference to get inside information.