I have a 8 month old that currently uses a convertible car seat.
Surprisingly, the infant carrier and rear facing convertible car seat take the same amount of room. We put the car seat behind the passenger seat, so the passenger seat is pushed forward quite a bit. I can just about squeeze in at 5'10" tall. Anybody shorter/with shorter legs can fit more comfortably.
We have a Chicco next fit zip, where you can zip off the cloth cover for easy cleaning and also bought a Britax Boulevard clicktight cool comfort for the CX9. Both are at the $300 mark and rate very high in consumer reports and wirecutter.com crash evaluations.
https://thewirecutter.com/reviews/best-convertible-car-seats/. The biggest differences between the specific Nextfit or Boulevard models are the ARB (anti rebound bar) and fabric types. So you just have to decide if you want the bar and pick which fabric you like the best.
I personally prefer the Britax after using both. Installing the Britax is much easier. It uses the car's own seatbelts and just requires you to push down (hard) on the car seat bottom cushion to get everything super tight. The Chicco next fit is also easy to install with LATCH, but requires you to push down on the seat while pulling up the straps as hard as possible. It doesn't feel as securely fastened as the Britax does.
The child seat belts are also better in the Britax. The seat belts and buckles can stay out of the way when placing the child in. The Chicco's belts are more difficult to keep out of the way, so you're always placing the child on top of the belts, then you have to fish them out from under them. One of the bad things about the Britax is that the button to release the child seat belts is very deep. You have to stick your whole finger in to reach the button. The Chicco's button is more superficial and easier to press. It's a small sacrifice to pay because once the button is pressed, the child seat belts are easier to pull out with the Britax.
The Boulevard cool comfort model means that the fabric and foam are more breathable than the other fabrics offered. So during the summer time, it'll keep the child cooler since the fabric is more breathable. This also means that liquids can more easily penetrate the fabric too.
The Britax shape also makes it easier to reach in to get a good grip on the child to lift in and out. The Chicco side bolsters seem higher, which makes it more difficult to reach in.
The build quality of both seats are really good, but it's just these little ergonomic differences that make me like the Britax one better than the Chicco. They are also both roughly the same size and can fit in the CX5 comfortably.
Pics:
Chicco NextFit iZip in my 2014 CX5
Britax Boulevard ClickTight Cool Comfort in my 2014 CX5
My front passenger legroom with a carseat installed. I'm 5'10 with a 30" pant inseam.