How old are the babies? How big and heavy?
The Britax seats fit in a wide variety of small vehicles (including the Chevy Cavalier, Pontiac Sunfire and even a Porsche 911
) because their design allows them to be installed pretty darn upright for older babies with good head control. They can also be tethered rear-facing to achieve a good recline and then braced against the front seat(s).
Most kids will outgrow the Marathon and Fisher Price Safe Voyage at about 45-55 pounds by height (shoulders over top slots), so the lower weight limit of the Safe Voyage Deluxe is not a concern for most parents.
The rear-facing tether and built-in lockoffs may be features you want, though. Installing a rear-facing seat with seatbelts that lock at the shoulder (I think your Subaru's belts lock this way) can sometimes cause a rear-facing seat to tip as the shoulderbelt gets tighter and tighter. A locking clip is the usual "fix" for this, but the built-in lockoffs are usually much easier.
The rear-facing tether is great for preventing the seat from rebounding (also called cocooning) and also helping achieve the desired recline level.
I'd definitely go with a Britax Marathon, Boulevard (same as Marathon but with true side impact protection via headwings and also a harness height knob adjuster) or a Fisher Price Safe Voyage Deluxe. These will be the easiest seats to fit in small spaces that will give you the most "bang for your buck" because they will last until kids are 4-6 years old and 45-55 pounds, usually old enough and mature enough to use a booster seat properly at all times.
The 40 pound weight limit convertibles are usually outgrown by height before 40 pounds, and at about 2-4 years old, depending on the seat. So parents either have to buy another harnessed seat at that point or put a too-young/too-small/too-immature child into a booster seat.