The Size 4 Me will actually likely last longer than the SmartSeat. The SmartSeat is not highly recommended because it is , is heavy, and can only be used rearfacing on the bottom 3 headrest positions. I wouldn't spend the money on it.
I agree. I just bought the Size4Me/MySize. My 50 to 51" 6.5 yo fits in the seat at its highest setting. She outgrew every other convertible seat by height over a year ago. This is one of the few convertible seats that will actually last until booster age.