What bit should i use on my mare?

I have been out of the show world for a long, long time and am just starting to get back into it. I have also decided that I would like to give western showing a shot, since i have a mare that has the training on her. Well she is just turned 5 years old so we have to move from the snaffle that we have always used (and will use for English) into a shank bit. What would be the mildest shank bit that i can put her in. She is very responsive to the snaffle so i want to move her to a very mild shank bit, Any advise?