Semigloss is less permeable, more moisture and dirt resistant and easier to clean. They use it a lot in kitchens too.
Just don't buy the cheap brushes, and you should be okay. If you are using oil paints (not so common indoors, but sometimes you'll use oil base for certain kind sof wood trim) buy the most expensive brush on the rack, you'll be glad you did.
For latex paints, an $8-10 brush should be fine. Keep it clean and it will last for quite a while.
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