Technically, a wire feed welder may or may not be a MIG welder, since a "true" MIG welder actually uses an inert gas (Argon/CO2 mix) to shield the weld area. But a wire feed welder with flux core is a reasonable facsimile, since the flux creates a gas shield (of sorts) as it it burns.
But my experience with both types is the MIG welder is FAR superior for sheet metal welding. My welder (a Clarke) came originally as just a flux core unit, and I wasn't very happy with the results (I had borrowed a MIG welder a while back to do most of the body work). I was just going to go get a true MIG welder, but I found I could get a gas conversion kit for my Clarke. And it works GREAT! The biggest advantage (learned from the MIG welding seminar I took through the Vintage Sports Car Club of Calgary, http://www.vsccc.ca/ for the curious) was to be able to use much thinner wire, and hence weld at lower heat with less burn through. The flux core wire was 0.035 " (thirty five thou) while the MIG wire can be as small as 0.023 " (twenty three thou). It doesn't sound like much, but it makes all the difference for sheet metal welding.
And I am going to split this thread, with the later part moved to the "Body" section.
A note of interest to some of the readers . Instead of paying good money for Argon gas for your wire feed, just ask for C25 it costs less and performs the same task as argon.( I pay $42.00 for a K bottle) Kwilford is very correct when he mentions the use of .023 wire , it definitely saves your butt when welding thin sheet metal. Also after you have switched from using flux core your welds will look 200% better.
don't go throwing that flux core out ,if you use it with the co2 it will give some good results.
I dont know if you have the same system in bars (you know them strange buildings with none drivers in) but i get my gas (straght co2 ) from my local pub for free as the brewerys supply it to them free and for steel its fine