I am a married Indian Citizen, me and my wife were on F1/F2 from Jan - Sep and converted to H1 starting October 1st.

I am filiing an extension so that I can pass SPT.

I was wondering if the estimated tax in the tax form needs to be computed using 1040NR or 1040 and does it matter?? As there is a substantial difference in the amounts that I get with these two forms.

Thank You

It DOES matter. Since you are extending to qualify for SPT, you will be filing jointly.

Use Form 1040 to compute your anticipated tax due.