Here's a list of seat recommendations from the Certified Child Passenger Safety Techs at carseatblog.com http://carseatblog.com/?page_id=401 and the forums at http://www.car-seat.org are a great place to get helpful car seat info from techs and concerned parents who spend probably more than their share of free time researching car seats.
Booster seats are safe and appropriate for children who have the age, size, and maturity to sit properly in one (the age at which this happens will vary from kid to kid.) For a 6YO, I think it *would* be useful for her to start "booster training" - sitting in a booster for short trips to get accustomed to sitting properly in a booster (and giving you a chance to reassure yourself that she *will* sit properly in a booster), because she's at an age where playdates and field trips become more frequent and (as much as we'd love to keep our kids in 5-pt. harnesses until they start driver's ed) you're probably not going to want to lug her Marathon and install it in someone else's car for a field trip or playdate.
The Graco Turbo Booster and Recaro Vivo are two high back boosters that generally fit smaller booster riders well. The Britax Parkway SG has a special "slide-guard" clip that makes the booster sort of a 3-pt. harness that could be good for smaller booster riders also - see review here: http://carseatblog.com/?p=2996. You want to make sure that the lap portion of the belt crosses low across their hips/top of thighs and the shoulder portion is crossing at the middle of the shoulder. This link shows some photos of what proper belt fit looks like: http://carseatblog.com/?p=3966
If you want a harnessed seat that converts to a belt positioning booster (these are called "Combination seats") - the most frequently recommended ones are the Britax Frontier (~$279, high height and weight harnessed limits, adjusts taller than any other booster in booster mode so it'll definitely last until your daughter can fit the seat belt alone - here's a carseatblog entry of a tech's 12YO using it as a booster: http://carseatblog.com/?p=5671), and the Graco Nautilus (slightly lower harnessed weight limit - 65 lbs, but plenty for your petite daughter to reach a suitable booster age, retails for around $170, converts to highback or backless booster). The Evenflo Maestro is a good budget option but will be outgrown as a booster at about the same time as it's outgrown as a harnessed seat. The other combination seats on the market are generally not recommended because they either have low harnessed height/weight limits, position the belt poorly as a booster, or both.