Dual shield is awesome for welding thicker materials that are not very clean. Flux core is more forgiving for less than perfect surface preparation, and the addition of a shielding gas (I use Stargon from Oxarc) makes chipping the welds easier and makes the finished weld very pretty. For wire I use Esab e71t-1.
Dual shield has been used for structural steel in fab shops for some time now.
Mig is my preference for most applications on vehicles such as frame, suspension, etc. The only thing I would caution someone who is new to welding with it about is the fact that it is much easier to have lack of fusion and/or lack of penetration between the weld and base metal because the wire will melt at a relatively low amperage and create a weld that looks "nice" to an untrained eye, but will not perform well. This can all be avoided pretty easily with proper machine setup and good technique, especially proper travel speed and stickout. Mig is also great for thin materials.
Flux core is a very versatile process that can be used for most vehicle applications. A couple of the drawbacks would be it's a bit messy, both in terms of spatter on surrounding surfaces and the amount of smoke it produces.
