what do you mean, welding a towbar? What thickness steel are you talking?
I'm pretty sure no matter what part of a towbar you are doing, the MIG will be far too weedy unfortunatly.
The arc might stand a chance, but I'd practice alot before risking injuring people on a motorway or similar if your welding failed.
With arc you need TONS of practice.
Do you have any scrap you can have a go on?