I've tried 'em all and the plasma cutter whups a$$ on everything. However, I do this stuff very frequently and for a living, so that justified the grand for the cutter (you also must have an air compressor).
So, a wealthy Santa aside, I've had great luck with a nibbler (if you have an air compressor, or maybe there are electric ones) that I got at Harbor Freight for like $20. It makes a mess with the billion little crescents it nibbles away, but so what.
My 12 Amp Hitachi sawzall is a beast as well, and as others have said, you'll use it for all kinds of stuff besides your car.
A good angle grinder with a slitter disc works well, but it's hard to get a nice straight line. It's also very useful for other stuff though.
Good luck!