I'm a long way from being an expert on bits, so I hope others will contribute here, but I'd be happy to get some discussion started. But I'll back up a step and provide some thoughts to consider before someone goes shopping for a better quality bit ... because you'll have a much happier and more responsive horse if you bring the right new quality bit home. ;)
I reckon the toughest part for the average rider is just defining the correct bit for that horse and rider situation. For most people it is trial and error. Not just snaffle vs. curb (I'm ignoring bosal hackamores and such here) but what kind of mouthpiece, what kind of metal (or combination), and design in general. One horse might go more comfortably in a 3-piece mouthpiece, and another might prefer a solid mouthpiece. A thinner mouthpiece tends to be less kind in a horse's mouth, but going extra fat is sometimes uncomfortable for the horse. Most horses go well in a medium thickness mouthpiece. Many horses prefer tongue relief, others don't need / want it.
In general [sweet] iron seems to be a favorite flavor with horses. Many horses like a little copper. Some horses like having a roller or something to play with when they're relaxed. Depends on the horse. And bit selection is also limited for some people by their discipline, where association rules of some kind will establish "legal" bit parameters, so people competing need to be aware of those restrictions. Training level is also a consideration, as is the horse's age and mouth maturity before any transition to a curb bit.
With shanked bits like the common [solid mouth] curb, a bunch of variables should be considered. A shanked bit is designed for use as a leverage bit. The most basic issues are the length of the shank, angle, and attachment .. and that needs to be considered according also to the kind of mouthpiece. Rider hands and horse responsiveness help determine the basic shank design. A fixed shank attachment is generally less forgiving than one that is designed to move a little, yet a shank that moves a lot creates a communication situation different than the common curb ... the rider needs to be aware of the difference.
Horse conformation and movement is a big issue, since even how the mouthpiece is set on the shank (high, low, forward, back) can change how a horse carries himself. (As an example, consider the elevator bit, with equal shank distance above and below the mouthpiece ... the location of the mouthpiece connection to the shank can really make a difference, and sometimes the difference is not correct for that horse.) Another conformation issue is the depth of the horse's mouth - where that mouthpiece would be carried correctly - in relation to the location of the chin groove. After all, part of the action of a curb bit is through the chin strap, so if that chin strap touches in the wrong place the bit will work differently and you may have a rather unhappy horse or one you're not communicating well with.
Snaffle bits have a similar situation to the shanked bit / curb in that there are different options for what the mouthpiece is attached to. Small rings, large rings, loose rings, "hinged" rings, different "ring" shapes, etc. They'll all talk to that mouthpiece a little differently on a direct rein communication ... and sometimes a difference in the bit attachment design will cause a difference in the response from the horse even with the same mouthpiece.
A shanked bit with a broken mouthpiece works quite differently than either a snaffle or a common (solid mouthpiece) curb. Some of these are often referred to as "nutcracker" bits. I figure if you don't very clearly understand how that hybrid bit works, and have a specific reason for using it, you shouldn't be using one. Then again, there are bits in the snaffle and common curb bit category that should carry the same warning ... or in some cases serve no good purpose in a horse's mouth. Also, many bits were originally designed for use as a temporary training device, not intended for daily use ... yet many are in daily use. So understanding the different bits and what they are intended to accomplish is another important part of bit selection.
The design of the bit is critical to bit selection for any horse and rider situation. Correct fit and adjustment in the horse's mouth is important - makes little sense to buy any bit that doesn't fit, much less adjust the bridle wrong so it is too high or too low. (Neutral position just touches the horse's lips.) And it sure helps if the horse has been taught to carry a bit correctly. Other issues include appropriate training with that particular bit for both horse and rider. And trying a new bit in a horse's mouth generally takes about 3 rides so the horse (and rider) has a chance to get accustomed to that new bit. But in some cases you can put a different bit in the horse's mouth and see an immediate change, if you know what to look for.
That's my fast overview of issues to consider for bit selection. After you've narrowed down to a certain kind / design of new bit to buy - or even if you're doing a trial and error "winging it" purchase - actual construction will make the difference between a "cheap" bit and one of decent quality. There have already been several posts discussing what to avoid in a bit. That may be easier than describing what to look for. I've compiled some of those "what to avoid in a bit" comments so they're all in one place.
Possible problems to look for - and avoid - in bits include:
poorly shaped mouthpieces oversized hoods (this is in the "traditional curb bit" category)
rough areas / poor finish work
oversized joints mouthpiece set in shank / cheekpieces too high or low
overly loose joints
poor quality metals and funky alloys
uneven balance mouthpieces
uneven balance shanks
non-round O-ring snaffles
places that will pinch
irregularities or discolorations in the metal
cracks in a snaffle joint or other weakness in the metal
Are all round pieces round and straight pieces straight?
Are shanks of equal size and shape?
incorrect weight and balance
I think the general summary of shopping for better quality bits - other than to avoid potential injury to the mouth with sharp edges and such - is to look for precision and balance in the construction ... not just something that is "close". Your horse will feel the difference, which will impact the consistency and quality of communication and response.
I was in the feed store the other day (not a tack store), thought of the discussion on bits, and looked at their little assortment of bits for sale. I found bits I wouldn't recommend anyone buy. The basic smooth snaffle had a 2-piece mouthpiece with one side longer than the other, and one side was thicker than the other at the outside edge. The common low port curb had the port rising off-center, and one of the shanks above the mouthpiece was bent out a good 1/4" from straight. I didn't even have to pull them off the wall, I could see problems without holding them for closer scrutiny.
It pays to learn about bits prior to any purchase, select something appropriate for the horse, rider, and situation, and be choosy about the quality you invest in. Your horse will thank you, and your wallet will appreciate making a single appropriate purchase instead of several wrong ones.
originally posted to the rec.equestrian newsgroup