1. What are the benefits of filling tax return as Resident Alien Vs. Non Resident Alien ?

2. If my wife is a non resident Alien while I am resident alien, can I file a joint return as Resident Alien ? What will the additional benefits of joint return ?

3. Do I need to add my Indian Salary also in the US tax return ? Can I choose not to show it as on it I am already paying tax in India ?

4. Can I claim my son as a dependent ? He was with me in USA for 172 days in the taxation year.

5. If I can't, still may I have any other tax benefits related to him ?

If you file joint return as residents, you can claim your son. You must both report worldwide income for the year. For your foreign income, you can claim foreign income exclusion Form 2555 or/and foreign tax credit Form 1116.
Other options is you file as married filing separately and your spouse as nonresident.
Compute tax both ways and choose what is best for you. Married filing jointly almost always is better. Your U.S. Tax Return: Moving Expenses (http://taxipay.blogspot.com/2008/02/moving-expenses.html)