i think a car sound system will always sound better than a home theatre system (depending on what is installed and who installed it)
but when your talking performance, you gotta leave it out. but that doesnt mean you cant have both....its just a handicap, as in, i can tell the difference when i have 1, 2, or 3 passengers...but its still "fast"(compared to my old hyundai:D)
but then a system in a hyundai will sound(and feel better) than a skyline!
its all about what you want your car to perform like and sound like, and how much mulla you have in your pocket at the time!!!