Do you have vehicle head rests? If you do you could try the cosco Apex. It is a 5pt harness that converts to a decent
booster, but must be used with vehicle head rests. It has fairly high top harness slots. This seat can be found at Sears for around $179.
Another option that would keep her in a harness longer would be the
Radian. It has a top harness slot of 18" but cannot be used as a
booster. It can be found at Canadian Tire for $199. This seat sometimes has install issues so you will want to make sure you can get a good install, maybe at the store or keep your reciept just in case.
There is the
Graco Cargo with upfront adjust that also has 17" top slots and makes a ok
booster.
The Evenflo Generations/Bolero have just below 17" top slots, and make a decent
booster. The Generations can be found at Sears for around $160 and the Bolero at Walmart for $120.
If you have the AlphaOmega right now, she should fit in all the seats that I have listed, as that seat has fairly low top slots. You might want to try sitting your daughter out in a few seats to see how she fits in them and you may even want to try installing them in your vehicle before making a decision. If you are concerned with the ways the seat installs in your vehicle then get it checked by a tech. You can call MPI to find a tech that is close to you.