I really hate looserings. They need to be fitted large (a 5" horse needs atleast a 5 1/4" if not a 5 1/2". My mare does best in a 6") to prevent getting pinched (if you think it doesn't happen very often, just stick your thumb over the ring hole and spin that ring and you'll see how easy it is to get pinched. Horse's lips are even softer and get pinched even easier), and they also need to be fitted higher to keep them centered in the mouth (even a bigger bit isn't any help when the bit slides as you use the reins and pinches the horse anyway), which puts constant pressure both on the lips AND poll, creating an insensitive horse.
I would personally suggest getting a french link. Most OTTBs are used to a simple broken snaffle, so those pressure points are familiar to her and she can ignore them. A french link changes it up and you'll find that she notices it more. I prefer Myler bits. There aren't any rough edges/bumps (which man bruise the mouth), and it can only collapse so far so it acts like a mullen if you have to pull hard on it, which puts more contact on the mouth for a stronger effect, but it's thin enough that the horse can't lean on it. (A lot of OTTBs lean on the bit).
I would suggest something that helps with steering, such as a fullcheek, fulmer, or baucher. D-rings are commonly used for racehorses and I would stay away from them for OTTBs. If her neck is stiff, I'd go with a baucher bit. (cheekpieces go through the small holes, reins go through the big holes). It puts a teeny bit of pressure on the poll when used, which helps encourage the horse to round, and it is pretty much impossible to pull through the mouth so it's awesome for power-steering. I love my Myler version. Expensive but soooo worth it.
Other than that, I love full cheeks. Again, go with a french link. Just make sure it isn't a Dr. Bristol link. They look the same except that the Dr. Bristol link is rectangular whereas the french link has more of a dog bone shape. A french link will also lay flat on a flat surface whereas a Dr. Bristol is tilted. Just ask us if you aren't sure.
Some people use "keepers" on full cheeks. It fixes the bit into place and it can't rotate. I don't use keepers. I think it makes the bit softer and more mobile.
If you like the action of a loosering, I'd go for a fulmer. Fulmers are traditional dressage bits (though they're hard to find), and they combine a loosering with a full cheek. It can't pinch the horse and also helps with steering.
Be careful with sidereins. Most of the times they encourage the horse to hollow the back and tense the muscles, which does not promote relaxation, correct movement, or muscle development.