If you shop at WalMart, they carry the Harmony Defender which has very slightly higher top harness slots than the Graco Nautilus (or Argos) and is also a 3-in-1 (harness to booster to backless booster). It's narrower and has a no rethread harness. It also only costs $100 but does not have the fabric options of the Nautilus. Some kids (esp heavier ones and boys) complain about the crotch strap of the Nautilus, so if you can I would try that out in advance. The Defender is known to have a short crotch strap but they will send you a longer one (for free) if you call them.
That said, it really depends on her torso height. The Nautilus and Defender are both around 18.5" top harness height and kids grow around 1-1.5" of torso height a year. Torso height (versus standing height) is going to be a much bigger deal in fitting in a seat. What size shirt does she wear? They get most kids to a size 6/7 shirt. You can also just sit her cross legged against a wall and then measure from the floor to her shoulder. If it's approaching 17", you may need a Britax Frontier (20" top harness height) to get her to a safe boostering age.
Most kids are fine in boosters by six, but if she's especially immature, you might want to take that into account as well.