• What’s a good score?

Below a IELTS band 6 is considered a fail.

• Which English Language testing TOEFL-IBT or  IELTS is better to get admission and TA/RA position in GRAD Schools?

Look at the particular university and program’s website. Relevant blogs usually have updated information.

For example, at the University of Minnesota, a TOEFL score of 23+ in each category is required for admission and 27+ in each category for TA appointments.