soft bits or harsh
You are asking a really great question, and I am sorry I don't have photos. But I can give you some pointers that will help.
First, parts of a bit:
The bar is the part that goes into the horse's mouth
The shanks are the part of the bit that lie against the OUTside of the horse's mouth
The barrel is where the bar joins the shank
Snaffle bits have no curb chain
Loose ringed bits have a ring that goes through the butt, or end of the bit - usually the cheekpieces of the bridle and the reins all connect to a single ring - there is a lot of play in this type
Snaffle bits work on the corners of the horse's lips, the bars of the jaw (where the horse has no teeth) AND - if too wide, too narrow, or adjusted too low - the roof of the horse's mouth. Which is why they wing their heads all over the place; it acts like a nutcracker on their jaw as well.
The fatter the bar, generally the kinder the bit. Bars that are twisted, barbed, have an edge, or are wires are NOT kind bits.
Curb bits have curb chains or curb straps and shanks which may be stationary or movable. If you look at a curb bit, the longer the shank is from the barrel (where it connects to the bar) to where the rein connects, the more action is on the curb chain/strap. This will make a horse lower its head, trying to get out of the curb chain's way. Horses with too much curb often have their noses tucked inward as far as the chest.
The higher the shank from the barrel to the ring the cheek piece connects, the more action will be placed on the poll. This is a very tender area for the horse. Horses with too much poll leverage will also lower their heads, but you will see them trying to relieve the pain in their head by dropping through the neck, and the nose pointed OUT. Pity the poor beast wearing an elevator bit with shanks as much as 6" above and below the bar - it basically works like a vise with the head stuck in the middle.
Rings on any bit allow for more lightness of action. If a bit has big rings, things roll around nicely, but still have limits. Small rings - or even rectangular slots - for either the cheek pieces or reins, quiet the motion, or add a stillness, to the bit - which may be good or not so good for the individual horse.
Looking for a kind bit will be looking for a fat bar, the width of which will be suitable for the horse. Whether the bit is a snaffle with one or more joints, or a curb, the fatter, the kinder. If the bit is a curb, go with the shortest shanks possible going down, and little shank above the bar. Try it with a leather curb strap first, then a curb chain - and make sure the chain lies flat.
Hope this helps!