I have the 3 QT Instant Pot for our RV for two of us. At home, I originally bought the 8 QT which comes in handy for company. Also, added a 6 QT to the mix at home for when there is just two of us. At home, we have more room in the frig to keep leftovers than in the RV so the 6 QT allows us an extra meal.
Cannot speak in regards the knockoffs. We are very pleased with Instant Pot. I agree with espendydad that there are tons of recipes written specifically to Instant Pot time specs which is handy. Knockoffs may have size and pressure differences that may change recipe timing. Also, lots of Instant Pot accessories available.