Hard to compare... completely different sims. A-10C is a full-blown study sim like the UH-1H model, with full systems modeling and a fully operational cockpit.
The aircraft in FC3 are not modeled to nearly that level of detail, and most do not yet have the advanced flight model (though that should be coming to the Su-27 and F-15C soon). Also, most aircraft do not yet have the nicely modeled 6-degrees-of-freedom non-clickable cockpit. It's an amazingly good sim, but it's a survey sim.
If you want to stick with A2G, I'd go for A-10C. If you want to explore A2A or mess with the older A2G aircraft, then FC3 is the way to go. Both are great options.
The one I'm really eager for right now, though, is the F-86F module... and the MiG-21bis if that one comes through.
I don't know much about the A-10A's weapon capabilities.