I just moved my 4 year old from his Britax Marathon (he is tall and thin.) It was a great seat that I used when he grew out of his infant seat. It allowed me to seat him rear facing until we switched him at 2.5. His baby sister now is able to use it rear facing. I was cognizant of the manufacture date when I bought it, since they expire in 6 years, so am able to use it for 2 children.
I moved my son to a Britax Frontier 85, which is a combination seat, it has a 5 point harness and will switch to a booster. It has one of the tallest shells on the market. It can only be used forward facing. It also has a 9 year expiration-so I may be able to pass it to the baby when she grows out of her marathon.
There is a great write up on the Frontier here: http://carseatblog.com/5751/britax-frontier-85-review-its...
It is a great site with a wealth of information!
Also, I purchased online from Albeebaby.com I agree that paying a bit more for safety in the car is important and albee offers great sales on Britax carseats. Also, I think they ship right from the manufacturer, so I didn't buy a carseat that was sitting on the shelf with an old manufacture date.
Good luck! I love our Britax!
Oh, you can also get the Frontier 85 SICT with side impact protection which may address your concern about your child being on the outside seats.