There's nothing wrong with showing and training in the same bit, it's just a matter of what's easier for you. While training, if you need to use two hands to practice or make corrections, it's easier on your horse's mouth to use a snaffle because, once again, curb bits are not meant to be used with two hands, and can become very harsh when they are. It depends on the type of showing you do, if it's like a state fair show or 4H you can show in a snaffle as well but for registered shows (like AQHA), I believe you can only ride in a snaffle in green horse classes. Also, of you choose to use two different bits, then make sure they're the same mouthpiece - you can have a snaffle with a low port barrel roller and a curb bit with a low port barrel roller, etc. This will make the transition from one to the other easiest on the horse.
I've spent most of my life riding horses. The rest I've just wasted.