Car seat dilemma

We have two kids, a 3 yr old and a 21 month old. We have bought 4 car seats (for the car) and we may have wasted money in doing so. :-/ As a guy who is known for researching product selections among friends and family, I can't believe that we may have screwed up our car seat selection this bad. Here is what happened...

Our first car seat for Anna was a Chicco KeyFit 22. It was a car seat that went up to 22 lbs. Before her first birthday, Heather was 3 months pregnant and Anna was not looking comfortable at 20 lbs in a 22 lb car seat so we went shopping for a convertible for her to move up in. At first we bought the highest rated one from Consumer Reports, the Evenflo Titan 5. It was a bit too big for Anna and the usability was horrible so we soon bought a Britax Roundabout and the Titan ended up in our second car for the past 18 months. When Madelyn was about ready to move up out of the Chicco to the Britax, we moved Anna to a Cosco Deluxe Summit which we bought. The Chicco has been passed to my sister in law who will soon be using it for the second time and will have gotten its fair share of use. Unfortunately, the Chicco KeyFit 30 wasn't around or it could have been used even longer.

So, there is the short story of how Heather and I have 3 car seats, the Evenflo Titan 5, the Britax Roundabout, and the Cosco Deluxe Summit. All three of those have one thing in common, their forward facing 5 point harness weight limit is 40 lbs. The Cosco has belt positioning, the other two do not. Why is this important? As of November, as Heather wrote here, Anna was 36.5 pounds. She was weighed again in January at 36 pounds...but how long until she hits the 40? Certainly possible before she turns 4...

Before we realized that yesterday Heather and I were blindly looking for another deal on the Cosco/Safety 1st Deluxe Summit to move Maddie up into. She isn't uncomfortable in the Britax Roundabout, but is more comfortable in the Cosco and Evenflo. Around the same time Heather saw the weight rating on the Cosco and we realized the mistake we had made, I saw this report on booster seats from the Insurance Institute for Highway Safety. To summarize, they think all the booster we have are horrible when it comes to seat belt positioning. Everything I read says to keep them in 5 point harnesses as long as possible. So, here are our choices in the next 6 months...

1) Buy a Britax Frontier which seems to be able to go on as a harness for much longer (80 lbs) and gets the IIHS Best Buy.
2) Buy her a Booster like the Maxi-Cosi Rodi or Recaro Vivo and switch her to belt positioning
3) Buy a 65 lb 5 point harness like the Safety 1st Go Hybrid and decide what to do in the future.

Right now I am leaning towards #1, buying a Britax Frontier. Thoughts?