Basicly, A-10A you will only use unguided weapons and mavericks and you will have to get low and dirty with the enemy, controlling is similar to SU-25T with keyboard and stick.
And A-10C, you get way more realism with clickable cockpit, you can use guided munitions (+the unguided ones the A uses) which you can launch from a distance away from harm with the use of targetting pod on exact coordinates, set up a flight network with buddies, share targetting info.
I use both from time to time.