I second the Britax Pioneer. I don't have personal experience with it, but we had a Britax Boulevard because I was convinced I would rear face forever. That lasted to age 2.5, when I switched her to forward facing. And then I was going to keep her in the harness forever.. until we didn't, and at about age 4.5 she was in a booster (Bog standard Graco Turbobooster, high back) even though at 5.5 she STILL can't buckle herself because of my car's floppy buckles and I now have to reach around her rather than just buckle in the front like we did with a harness. The Britax Pioneer (or Frontier) would let me continue to use the harness.
I will say that the more expensive seats have more convenience features. Given that it's something I use every day, multiple times/day it was totally worth the extra $100 to, say, have a smoothly tightening harness, or to not have to re-thread the harness when she grew taller. We had cheap seats at the grandparents' or from rental car companies, and always appreciated the Britax more when we got home. As the first reply said, it's an awkward time to be spending the money because at 3 they'll outgrow the car seat and move into a booster soon. Since the Pioneer is both it makes sense to me.
edit: The cheapest equivalent would be the
Cosco Highback 2-in-1 DX Booster Car Seat, $45. One set of grandparents has this, so I've used it and while it is functional there are pain points. For example you have that re-threading problem with the harness as the kids grow, the LATCH clips aren't as easy to use as the Britax ones, the weight limit is much lower than the Britax (Cosco only goes to 40 pounds with the harness, Britax to 80 supposedly), the cupholder is flimsier.