Ditto the riding in back with him sometimes! That's how I drove across the country with a 3 month old infant who DIDN't sleep in the car (we got 2-3 hours napping out of 8-10 hours driving if we were lucky). My back was a mess though afterwards. However, if sanity saving and no crying makes for a safer, less distracted driver, then it is "legal" to forward face in Canada at 12 months. Either way, the middle back seat is the safest spot.
This is unrelated, and don't mean to scare...but please also have a cargo barrier to prevent objects in the back of the Matrix from flying into the passenger area. I find it ironic how there is so much focus on choosing the right carseat (in case of an accident)....yet so little discussion exists on the topic of cargo and how the safest seat cannot prevent a head injury from a flying object (even a water bottle can seriously injure). Even the safest of SUVs with the latest airbag technology can't stop a can of chick peas from becoming a deadly weapon in a rollover.
The Radian is 17" wide on the upper part (same width as an Alpha Omega) and gradually narrows down to the seat that's narrower (I think about 15"?) which makes it so much easier to fit/install and still be able to find the other seatbelts!(...if that's any help seeing how it might fit in the middle back seat rear facing). Also, it sits lower to the seat (whereas Britax is on a tall base) which makes it easier for the driver to do shoulder checks, plus allows it to fit a little more easily rear facing than Britax in a small car (the back of radian seat would contact the passenger seat a bit lower down). I have both seats now....I loved the Britax when dd was a toddler (being higher up rear facing so she could see made all the difference in her enjoyment of travelling)...but now I love the Radian now that she's forward facing and getting close to outgrowing the Britax MArathon's height limit. I love that I can keep her in 5 pt harness in the middle back seat of our 2006 Ford Focus wagon (similar sized back seat space to a Matrix) and still have room on either side for a carseat or passenger (expecting an infant this fall and will be getting a Chicco Keyfit seat so I can still ride comfortably in front passenger seat).
While I love the Britax, basically the Radian would be more versatile over the longterm...and an 8 yr expiry date (vs 6 years for Britax) would mean your second child could use it also for a long time.