I would have to say that flux corewire is ideal to use on thicker materials. We, generally use .045 diameter wire. Definately 75/25 argon/CO2. We pretty much done away from the hard wire because of its lack of penetration. On the thinner materials it works great on filling gaps. Usually I weld thicker materials around a 1/4 inch or more. Flux core reigns over hard wire. Flux core gives you alot less of the "sizzle" and alot better looking weld because of its superior coverage. Alot, thanks to the flux thats in the wire core. Its more costly to go with flux cored vs. hard wire, but then again, your end results speaks for themselves.

Jeff