Hi, H.. I work at a place that sells baby products including carseats. Let me start off by saying that just because it's more expensive, that doesn't make it safer. Carseats are my department...Britax is rated higher only because they test their seats at a higher MPH than the rest of the manufacturers. And it's a 5 MPH difference. That can make a difference but most parents don't have the budget for how expensive those seats are. My first suggestion would be to bring your child into the store and put him in the seat. Buckle him in, take the display seat to your car and see how it fits (if the store will allow you to) and let him tell you if he's comfortable. My second suggestion would be to try the Alpha Omega Elite convertible seats. There's a few different colors/fabric patterns to choose from. What I think is so great about this seat is that it goes from 5 lbs to 100 lbs. And, with expecting another child, it does go rear-facing! It has a 5-point harness with an adjustable back for height, and once he reaches the state requirements for height and weight to use the car safety belt, this seat has guides the belt goes through so you can use it just like a standard booster seat.
If you have any further questions, please let me know. The store I work at has the Marathon but not the regent so I'm not positive on the differences between the two. The other Britax seats we carry vary with weight accomodations and, of course, the pattern on the fabric.