I would like to take my master degree in USA. I need to know, should I go with power system engineering or control system engineering. Which one is better in nowadays.
That's your choice, it will be your training. There's always a mystery factor to this decision because nobody really will know what will be the next "hot" field. Also often what gets predicted to be "hot" then draws an excessive number of students and bingo, in two years a field that had not enough graduates now has too many.
So choose the field that intrigues you. Enter the finest school that will both accept you and that you can afford. This will maximize your grades and prestige in the field you enjoy.
Good Luck.
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"Don't disturb my circles." translation of Archimedes last words
When given that option I decided on controls as they are ubiquitous and cover a large swath of applications, power implies you'll probably wind up working for some utility company (not that that's a bad thing). Choose whichever one that will make you want to go to work in the morning and you'll be fine. Good luck.
If you are interested in a field and become good at it then you will probably be successful even if it is not the "HOT" field.
If you do not excel at something then you could, at most, only expect a few good years in a "HOT" field. Chances are that without a strong interest you will fail even if the field is "HOT".
Most people that are good at a technical job think about technical things while driving, eating, mowing the yard, etc. What other people think you should become is not the right question. You should be asking yourself what you want to become.
Notice that I didn't say "what classes you should take". My statement was "what you want to become".
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Few things limit our potential as much as knowing answers and setting aside questions.