At home, flat and jump in a plain loose-ring snaffle, which shouldn't make your horse hard-mouthed or overly sensitive. If he tends to be "pull-y" and get a little on his front end, you can try lifting him a little with a gag, but only a gag snaffle-which, again, isn't hard on his mouth.
School at the show in your loose-ring snaffle; then move up to something that's more suitable for your horse to ride around the course. I always like to start with a plain snaffle-because if you use too much bit, you can easily end up creating a hard mouth.
But if your horse is a little heavier, a little stronger, move up from there to a slow twist, or something slightly stronger-whatever suits your particular horse. If he takes lots of work to get going, and he just naturally slows down the moment you take your leg off, the last thing he needs is extra pressure from his bit.
Practical Horseman, April 2001