Should i buy a brand new car or a used car under $10k?

With the used car, you owe no more money on it.

With the new car, you DO owe more money over the $10K.

As no one here knows your income and financial means, no one can even GUESS what you can afford.

You know....if you can't figure it out by yourself and feel the need to ask strangers perhaps you should buy a $3000 beater to get around in until you figure out what you're doing with your life.

You buy the car that you can afford.  If all you have is $10k right now, get a car loan.  You can be unemployed tomorrow.

It is your money.  You can do whatever you choose.  I personally would by the car under $10,000,   In three years a newly purchased vehicle is worth about 40% less, and you end up paying a lot more with the interest payments.

The most economical thing to do is to buy a late model used car.  On such a car you lose most or all of the new car warranty, but you will also be paying about half as much for it.  That $250 is going to mount up $9000 by the time you pay for the new one.  

  3 years from now, would you rather have a 3 year old car, or a 6 year old car and $9000 in the bank?  The longer you keep the car the less difference in value there will be.

   Best to buy the used car, but do try make sure that you buy a good one.

we have no idea what your situation is.

how would we know if it's better to have a new car or a used car.

Avoid an auto loan.  Pay cash.  But a Honda Fit or Toyota Yaris. A 2013 model with less than 100k mileage.