Here is an article I wrote on the subject and posted here on LilBeginnings a couple years back:
Sidecheck vs. overcheck
There is not a lot of difference in the checkrein styles if your horse isn't checked up very high. For every day pleasure driving or for the Country Pleasure classes, the horse shouldn't have a lot of contact with the check for him/her to really feel it and for it to have any adverse effect on how the horse carries his head.
Now, when you get in the classes where your check is rather tight, the style of check may affect the horse.
Consider the placement of the checks:
The side check attaches to the bit below the cheeckpiece, and runs up to the ear. When the horse hits the check, there is a direct pull into the horse's mouth, the bit will pull towards the molars if that makes sense. The horse's nose will freely be able to swing at the throat pivot if that makes any sense.
The over check runs between the ears and down the face and attaches to the bit above the cheeck piece. When the horse hits the bit, there is a slight pull UPWARD in the horse's mouth, not towards the molars quite as much but more towards the roof of the mouth. The horse thusly will be a little less inclined to swing it's nose towards it's chest, unlike the sidecheck which allows for freeer movement.
That said, you will have to consider the individual horse and how it carries it's head and neck, and how it's headset is.
I like the sidecheck for most horses, particularly green ones. It does not impede their headset as their nose is free to swing whereever it likes. It is a fairly comfortable check as horses don't get their heads wrenched up as high with a side check. It helps horses learn to "tuck" their heads because the overcheck sort of works against that. And "just for looks' sake" the sidechecks tend to "look prettier" because you don't have all the leather straps all over their face smile.gif
However, some horses go better in an overcheck. The overcheck helps pull the head up higher than the sidecheck if you are looking to check your horse up very high (as long as the horse is properly built with a high enough neckset!). Some horses tend to get behind the bit (they tuck their nose in too much and the nose goes towards the chest) and an overcheck is more ideal for that situation. You will see overchecks a lot in the draft horse breed, where picking the head up HIGH is more of a priority than setting the head, and in breeds that are BRED to drive like hackneys, where they are BRED to have a super high neckset and an extreme headset where an overcheck doesn't impede the headset due to the horses' natural conformation.
I actually think that conformation plays a big part in check selection, because how they carry their head naturally will depend on how the check will aid or hinder what you are working for.
So there you go, I don't know if that helps you any. It's a minor selection, but you may find a horse goes better in one than another. I have a friend who drove in an overcheck and fought a bit with the horse... they switched to a sidecheck and the horse went happy as a clam.
Those are just some of my opinions that you may want to consider. I'm sure there's more I haven't thought of , I am interested to see what others have to say.
ADDED NOW: It was brought up later in the post to additionally check the show rules for the classes/registry you are showing. Some driving classes require NO checks. In the Shetland Pony classes it specifices which checkrein to use, for instance I think it's overchecks only on the Modern Roadster ponies.
Andrea