DC-DC is the most effecient and will allow your computer to produce highest quality sound. However, also being a high school student I know about being strapped for money and time so I don't see why you wouldn't use and inverter that you already have. It's not like it will cost you anything to switch in the future.
Got started with computers on this board early in high school...now I run a web development company and am about to build my dream machine.
DC - DC is the best you can do for your project, b.cause eficiency and low noise. But if you´re already have an inverter go for it. just ground it well to the car and see if noise comes up to the sound.
I'll say this I had the inverter and it worked pretty well. I upgraded to the atx dc to dc power supply and I can honestly say I would never use an inverter again. Once you try it you can never go back. It might have cost $170 bucks but it was worth every penny.