I have a 3yr. Old Thoroughbred/Draft and she has been ridden english since I started riding her October of 2011 in an oval mouth copper loose ring snaffle bit. I want to start her western but am not sure what the best bit would be to use.
A Snaffle bit is fine. It depends on what kind of western riding you want to do if you want to do western pleasure or riding, a crub bit is traditional, but Snaffle bits are good for young horse because they are softer on their mouthes. Hope it helps :)
Just a tip, if you're thinking of switching her over to a curb bit, don't go straight for the tom thumb. So many people do this when switching a horse from a snaffle to a curb thinking it's similar, but in fact tom thumbs are quite extreme curb bits reserved only for people and horses who really need it and know how to use it. Though I'd prefer it just never be used at all. If you do decide to switch your horse to a curb look for something with short shanks, a moderately long purchase, and lots of tongue relief. Give your horse plenty of time to get used to the new bit and remember, don't direct rein in curb bits, it's not really designed for that. So make sure your horse neck reins well before switching bits.
This is my favorite curb bit, it's called a sweetwater bumper bit, honestly I find this more gentle even than snaffles when neck reining:
I have one of those bits punks...My 2 broke horses HATE it. I worked for a trainer, and when he would start bringing a horse out of the snaffle, he would use a correction bit ( has shanks and a curb, but is still double jointed and has jointed shanks) before going to a curb, so the horse could get used to poll/ palate pressure while still having direct reining capabilites to fall back on.