Is she staying over 40lbs? And is she mature enough to stay in position in a booster the whole trip every trip?
I'm not a fan of the term "booster training." It's a concept you hear around here, but really, a kid reaches a point of maturity where they're simply ready for a booster, and before that they're not ready. You can put them in one and coach them, but if they're not ready to ride in a booster, repeated booster trips aren't going to make them develop that maturity.
The most important thing when you're looking at transitioning to a booster even part time is to understand that when you place them in a booster you are placing your child in a position of being responsible for her own safety. With a harness, you're responsible for her safety.
If you think your daughter is mature enough to take on that responsibility from time to time, then you can try her in a booster and see how it goes. If not, then it's best to wait until you think she is ready.
So far as a booster that fits small kids well, I'd look at the turbo-booster. It can be a bit harder to buckle because the armrests are fixed and you have to make sure the shoulder belt is routed underneath the armrest on the buckle side, but it tends to fit the small booster riders well. Whatever you choose, you most definitely want to have a highback booster for her.