Simple answer. WIth an inverter is what basically happens is you put a standard plug outlet in your wall, into your car. IE you can use the PSU now. Downsides is they dont always work with every computer/psu for some reason, they generate a lot of heat, and dont come packed with the features ones like the OPUS do (such as startup/shutdown etc.). Also just not as "cool".
DC-DC has a better reputation as cool (and come on CarPC all about the bragging rights haha) is more efficient, and generates much less heat. Downside, your limited at this time to 150W when buying, you can make a 250W one thats talked about here but it looks kinda complicated (though a ton of people love it and had no problem) and I think a 350W homemade one is in the works as well. Or if your lucky like me you find one of those older (used) retail 250+w DC-DC that people used to sell.