The Tom Thumb is a bit that gets a bad rap for being very harsh--stories have been passed around of people breaking their horse's jaw with it. Now, that seems a little exaggerated to me (I can't imagine how much pressure they'd have to be exerting on the reins to cause that much damage), and while I don't consider the TT to be particularly cruel, I just plain don't like it for a variety of reasons. Many people do use and love this bit, swearing by it and using it for training colts. To each his own, but my personal opinion is that there are far
better bits on the market.
This, right here, is a true Tom Thumb--and the worst one in existence. The straight shanks don't give the horse any warning at all--the second you apply pressure, they pop right around, snatching the curb and activating the nutcracker effect with the mouthpiece. It also makes direct reining difficult--the straight shanks, combined with the way the mouthpiece attaches to the shanks (no freedom whatsoever), mean that when you pull on one side, the entire bit twists rather painfully in the horses mouth. The curved shank versions aren't quite as bad, but it's still quite difficult for the rider to convey the intended cue to the horse, no matter how light or heavy his or her hands are. This bit just doesn't have a whole lot of finesse in the horse's mouth--it's all or nothing. And any efforts at direct reining will likely result in confusion for the horse. While many horses do ride well in a TT, this often has far more to do with the kindness of the horse's temperament than the quality of the bit.
Now compare the above bit to this, your standard Argentine Snaffle.
At first glance, they look very similar. But the Argentine is much better balanced. Look at the curved shanks--these make it less harsh, and a slight touch of the rein will give a horse warning that a cue is coming, giving the horse time to react. Also, the joint between the two parts of the mouthpiece is finished better, so it will be less likely to pinch. Most importantly, the attachment from mouthpiece/shank is not fixed in the vertical direction. This gives both parts some freedom and independent movement. Slight jingles in the reins are often all that is needed to give a cue and thus get the desired response from the horse without the yanking that is often required with a Tom Thumb. Ask any horse--most will respond so much better to bits of this type.
Of course, the "snaffle" mouthpiece in both bits has the potential to "nutcracker" and pinch, so be careful with that. A three-piece mouth is more desireable in most cases.