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"Advantages of tig would be the pretty weld."

so nothing to do with one being ac the other dc, the availability of different filler metals and the different temperatures involved or the fact that the filler wire with the mig is used to deliver the current whereas tig uses a tungsten electrode.
there are other differences but i haven't the time to phone my old man and ask (he used to be a welding demonstrator/technician)

no this is a fixed gear and hence all about pretty.;)

Seriously though. I know there is stuff like more concentrated heat but don't know the details if you know them post them. The filler wire delivering the current I think the biggest problem this brings is more likely for some porous material to form, but if and once setup like it would be on a production line I think this can be minimised. AC vs DC I don't know about I think it is similar effects on the forming of porous area. I think the forming of porous areas is the reason why mig welds tend to be chunkier than tig to compensate for this. I know aluminum is welded a/c tig or mig.

The important thing though is that the properly set up the joint is stronger than the surrounding material so any failing in the frame will not be at the joint. This tends to be the aim of most metal joining processes.

Your correct to pull me up my statement. Any extra information is always greatly received by myself.