I'm not savvy on tack but I'll try and explain it correctly. I have an 8 yr old mare I trail ride. I use a snaffle bit. My farrier said to get a D ring snaffle. I've always used the snaffle with the shanks on the outside of the mouth of which he said was too severe for an experienced horse. What's the difference and why is one used over another? Also, do I use a chin strap with the D ring? The farrier said I didn't need to. Any advice would be appreciated.
A curb too severe for an experienced horse??
Curbs are only for experienced horses, and definitely not for "training".
Snaffles have direct pressure to the mouth. No leverage.
They're for direct contact, and the bit to start with for schooling.
For you, trail riding, a snaffle is fine.
Curbs have leverage. Depending on the shape and length on the shank, the pressure is intensified, and therefore is meant only for a finished horse that neck reins and goes off seat and leg. They're not for schooling.
Now, snaffles are commonly referred to as the single jointed mouthpiece.
That's false. What defines a snaffle is the rings (dee, loose ring, eggbutt, full cheek, etc.), not the mouthpiece.
There are singe jointed curbs, also not referring to a tom thumb, as that's something different in itself.
Bottom line, your dee ring is fine for trail riding, and only if your horse can independently neck rein one handed should you use the curb.