Which bit type should I use?

I am not an expert but here is what I think - number one, curb bits are usual in western riding, only recently used in western riding (I don't know which you ride). Second, you have to start with the gentlest bit, as your horse has quite a tender mouth never having a bit before. So you should start with the snaffle unless it turns out that she needs something more severe. There are also several different types of snaffles, some more severe than others. I would think the trainer would know (I would hope so) unless the trainer normally just uses curbs. Trying a snaffle cannot hurt. Once a horse's mouth is hardened, a lighter snaffle just does not do the trick...start with the gentlest.