First question; has this horse ever been ridden in a bit before? If so, what level of training had they had?
If the horse has never had a bit on before...or if they had only ever been ridden in a snaffle before going to the hack, then I'd start with something like this Amazon.com: JP KORSTEEL Copper Oval Link Loose Ring Snaffle Bit, 5.5": Sports & Outdoors
Then, when they are trained enough and ready to move up to a curb type bit, I'd go with something like one of these...
I have bits identical to these and me and my horses both really like them Amazon.com: Francois Gauthier Antique Hinged Futurity Bit: Sports & Outdoors Amazon.com: Half Breed Sweetwater Mullen Trainning Horse Bit: Sports & Outdoors
This one is very similar but has a full copper mouth instead of sweet iron with
Copper inlays Amazon
Coronet Training Short Shank w/Copper Wide Port Bit - 5": Sports & Outdoors
As for your original question about the "harshness" of the broken mouth curb bit (it's not a snaffle with shanks, for more information, you might want to read this thread Bit Information (Curb and Western type bits and hackamores)
), it's not so much that the bit is harsh if the hands are good, but the bit is just ill-designed.
I have tried a TT and, while my horse didn't have a problem with it (I can ride him in just about anything), I hated the lack
of feel that it had. There was no subtlety with it, no nuances to my cues, it was all very blunt and rigid. I can compare it to trying to put in stitches with a sharp stick and kite string. You might get the job done if you really
know what you're doing, but it won't be pretty.
I greatly value the amount of feel my horses have and I expect a great deal from them. While the bits that I generally use aren't those big, expensive, custom jobs that some do, I make sure that the mechanics of my bits are suitable to the horses that I ride and the goals that I set for them.
This is strictly a personal preference, but I thoroughly dislike bits that have shanks and a broken "snaffle" mouth, just because I don't like the feel of them. I much prefer a good ported curb, either solid or with a barrel. I like being able to pick up one
rein to fine tune a movement without the entire bit collapsing on the horse's face.