Re: To Booster or Not to Booster?
If she still has growing room in her harness, then there is no need to rush her into a booster. Except - a situation might come up where it would be convenient for her to use a booster, and if that is the case, you'd want to know ahead of time that she could behave while sitting in a booster. So it wouldn't be a bad idea to think about getting a booster and giving it a try on a few short trips to see how she does.
As far as best boosters go - as long as it properly positions the seatbelt on your daughter, we don't really have comparative safety ratings. That being said, personally *I* have a preference for LATCHable boosters with big sides. The Recaro Probooster looks super protective, and I think it is going to be LATCHable soon, if it's not already.
The Graco Turbobooster that a pp mentioned generally fits kids well (especially small kids), is light, readily available, and inexpensive, so that's a nice choice as well.