What bit should i use on my mare?

In order to make the transition, you should use a Pelham with double reins. This allows you to use the snaffle reins for all direct reining while adapting the horse to responding to engagement of the curb reins.

I use a mullen mouth Pelham so that the horse is also learning to accept the feel of a solid mouthpiece with signal coming to both sides of the mouth at the same time, so that when you go into the finishing curb bit, you can use a low or medium port (or mullen) mouth curb. I also never would engage leverage shanks against any jointed mouthpiece as it is too painful, and completely confuses your signals.

Before you transition, your horse should be neck reining well for all lateral work in the snaffle, and slowing and halting on body aids. A shanked bit should never be used for direct lateral rein cues since the bit will twist painfully inside of the mouth. The purchase will apply pressure to the same side of the muzzle as the active rein, pushing the face away from the direction you seek, and giving the opposite signal to the one desired. This is why it is important to have the horse neck reining before you transition into the curb.

So, I'd go with a mullen mouth Pelham fitted snug at the corners of the mouth (no wrinkles and bit the same width as the mouth) to start transitioning, as long as she is neck reining and responsive to body aids.