When I was picking out transition bits, the second one that Joe posted was one of my top picks. However, like you, I prefer a bit with copper and/or sweet iron, so I chose this instead:
Western SS Hinged Ported Roller Bit - Horse.com
Awhile ago one of the trainers at our barn happened to see it on my horse and mention that she used the same one to show all of her training horses in since it's a very easy transition from snaffle to curb. It's a nice, forgiving bit with completely independent sides for occasional two-handed reining. My horse LOVES this bit. It's good looking, to boot!