i got a gs300 rear end is300 5spd lsd rear end and gs400 rear end which one is better for my 2000-gs300

 

Is your car turbo'd? I'm assuming no. In that case just leave what you have. The GS4 rear end is geared lower and that will make you accelerate slower, but it will give you better fuel economy on the highway. The IS300 rear end does not bolt up to the GS.