We have a Britax Diplomat, and it fits rear-facing in our Chevy Cavalier. The Roundabout is the Britax's other "compact convertible." The difference between the two seats is the type of side impact protection. We went with the "True Side Impact Protection" in the Diplomat.
We'd been using the Alpha Omega Elite, which I've really loved for both our kids - and we could buy 3 seats for the price of one Britax. The only drawback is that it only works with the 5pt harness for up to 40 lbs. And now they are recommending keeping kids in a harness longer than that..
So, I just bought the graco nautilus for my DD - She just turned 3 and is about to hit that 40lb weight mark so the Alpha Omega would just be a booster, and I'm not quite comfortable with that just yet. But, my DD IS on the chunky side, so I think she's hitting that limit earlier than most kids..
So, my recommendation is to go ahead and spend the money now on a seat that will go to 50-60+ lbs with a harness. That way you are covered for longer.