Snaffle. The only time I reverted to a tom thumb was when I was 14, stupid and given a runaway horse to train. It worked in getting her to stop, but it sure took me a lot longer to soften her mouth up again afterwards and have her go nicely in a snaffle.
Right now I'm riding my almost 3 year old Paint filly in a loose ring french link snaffle. She seems to love it. We've worked on whoa on the ground and whoa in the saddle. It was one of the first things she learned. I ride her mostly on a loose rein, and I can darn near pull a sliding stop out of her just by sitting deep, and saying "whoa". I don't need the reins at all to stop her. That is the type of horse that becomes a candidate for a curb - a horse that can be ridden on a completely loose rein and barely needs the bit to communicate. The curb offers refinement in the showring, so that a twitch of the rein is all that is needed as reminder.
I believe it should be mandatory for all snaffle horses to be ridden in a specific pattern without any bridle before being moved up to a curb. Only when you can control them with virtually no headgear should you actually move up to a more refined bit. I hate the term harsh, because the entire point behind the bit is to NOT be harsh. A curb is not a training device, it's a finishing device.