Generally, the harshest snaffle would be twisted wire: snaff.jpg
Mostly because it's wire is thin, and the thinner, the more pressure is exerted in one area (soft sensitive toungue).
When going to get a good snaffle you will want one that is thick and will not pinch when the nut-cracker action is applied.
And there are several ways to tell if a curb is too harsh.
I will use this curb to break down its parts: high port bit 2.JPG
The yellow lines are the purchase. You will want to aim for the shortest purchase possible. The longer the purchase, the more pressure applied to the poll (top of head).
The red is the port. Typically, the lowest port is most desired, but whe too low, it creates no tongue relief for the horse when you release the reins. So really not so
good. It would be better on the roof of the horse's mouth, but hasn't as much toungue relief. A higher port (like on this one) creates plenty of tongue relief when used correctly.
Blue lines are the shanks. Generally, the shorter the better. But the longer the less pressure you will need to use on the reins.
This would make an ideal snaffle: snaff 2.jpg
This would make an ideal curb: curb.jpg
The shanks are a tinch long, but if used correctly and with a curb chain not strapped on too tight, it should be fine.
Try to stay away from snaffle-mouthed curbs like the Tom Thumb: tt.jpg
Extremely harsh 'nut-cracker' action. Today's Horse - The Trouble with Tom Thumb
A bit is generally only as harsh as the rider's hands, but bits like these aren't gentle nomatter how
you use them. Only if you don't use the reins completely. Then why bother using a bit at all? badddd.jpg
Hope I helped :)