All of the snaffle cheeks are good bits, and the differences are subtle. For most horses they don’t matter and for most cases you can go with what you have or what you think looks good on the horse. Fashion for “what looks good on the horse” has definitely changed over the years and anyone telling you that a particular bit is “traditional” is probably wrong.
My pony goes best in a bit with cheeks of some sort: D, full cheek, or eggbutt, because she can sometimes resist sideways and because she likes a bit that doesn’t move in her mouth. Her favorite is a Duo flexible mullen D, but she tolerates a metal mullen eggbutt snaffle. Many horses I’ve ridden are softer and more comfortable in a loose ring, but not her; she despises them and for that matter anything with a joint.
There are small differences in the physics of the way that the mouthpiece will rotate. With perfect round rings, as in a full cheek with keepers, the mouthpiece will not rotate when the reins are pulled. A D-ring is one where they may rotate more, depending on the shape of the D. The shape of the mouthpiece against the mouth may or may not change with this rotation as well. Loose rings tend to be very simple and very symmetrical. The more curve there is in the mouthpiece, the more a potential rotation will matter.
If you want to see this, you can put the bit in your hands, hang it, and pull on it with reins to see the sweet spots of where the cheeks and reins end up when there’s tension versus not. The height you hang it at on the horse (the ever popular number of wrinkles) will impact this a bit as well.