Personally, if I'm going to use a curb bit, I prefer it have a slot so that I don't have to run the chain/strap through the purchase hole/ring which can pinch the horse (the hole/ring that the headstall attaches to - like the examples in Cowchick's photos - every single one shown uses the the same hole/ring for both headstall attachment and the curb strap) If the tack is properly fitted and you are using it properly, I honestly don't see how a horse can be pinched. I've been using bits like that (with just the one ring) for decades and I've never had a horse get pinched....but maybe that's just me .
And in Smrob's post every bit shown/linked to, with the exception of the last one, are TWO rein bits with NONE having a separate slot for the curb chain/strap. Even though these bits are designed for 2 SETS of reins (one ring at the mouthpiece ie. "snaffle rein" and the other at the end of the shank) you almost never see 2 used. Yes and no. They are designed so that 2 rein sets can be used but there is no hard and fast rule that says that is the only way to use them. Matter of fact, there are very few people that even know what those rings are for, most simply think it is a different way to keep the mouthpiece stationary on the shank and prevent a gag action. We're not talking about a dressage double bridle or a bosalita and spade combination here where you need to have 2 sets of reins to make everything work the way it should. I like bits with the snaffle ring because they give me the option. Do I use it every time? No. Do I use it often? No. But, occasionally, there comes a horse where you do need to use the snaffle rings for a few rides until they can get used to the different pressures and feel of the solid mouth before adding the curb pressure as well. Just because I don't use them every time doesn't mean that I am using the bit improperly
If the horse is properly prepared in the snaffle bit, then bumping up to the curb bit is often no more complicated than just putting it on them and taking 5 minutes to get them used to the different pressure points and the different action. If the horse suddenly
has a conniption fit just because the bit is changed, then either the rider doesn't know how to properly use a curb bit and is hurting the horse by pulling on it too hard or the horse wasn't properly prepared to respond to other aids that accentuate the cues given through the bit.
As for the balance thing, I have to agree with Chick. You can get a great idea of the balance by balancing it on your fingers. That will show you the natural balance of the bit so that you can match it to the horse's headset and ability. You wouldn't put a bit with a straight up balance on a horse with a more nosed out headset and you wouldn't put a more swept back balance on a horse with a straight up headset. Doing either of those things would make the horse uncomfortable and more likely to act out because the tack isn't matched to him.
I did want to touch on that first link that Horse Poor posted, those people only take into account the shanks on the bit and they make assumptions about a horse's headset and other things that can get a person into trouble.
1) A bit does not create a vertical headset on a horse and if you are depending on a bit to teach that, then you aren't a very good horseman.
2) You will never find a good quality bit that is, as they say in the article, "overbalanced". That bit is a catastrophe waiting to happen because every time you pick up the reins, there is no warning of the cue to come, it all just comes crashing down into the horse's mouth like a ton of bricks. The only way that would be balanced bit on a horse is if the horse's natural headset was "rollkur"
3) They assume that a bit that is "underbalanced", or has swept back shanks is undesirable. That's only true on a horse that doesn't
have a headset to match it. On a horse that travels slightly nosed out (like 90% of horses do), that would be a well balanced bit.
Those folks have their facts backward on the signal time and pressure build time. A bit with more swept back shanks gives more warning to the horse than does one with straighter shanks if ridden on the same length of rein.
The straight shanks are one of the problems with the Tom Thumb bit and one of the reasons why it's an ill-designed bit.
And so, we come full circle.