There are no studies showing the tether or airbags on the side of the Advocate help. All seats pass the same testing. If you're more comfortable with one versus the other that's fine, but just know that non-tetherable seats are perfectly safe. The most important thing to do is to have your younger child rear facing as long as you can, and use all of your seats properly. Any seat when used properly is very safe.
The NextFit is much taller than the Britaxes rear facing, offering about two more years of rear facing time. She's not heavy, so you don't need to worry about the 40 pound weight limit. If her torso is longer (her shirt size is bigger than her age) the Britaxes are generally not recommended as they get kids to about a 3T, so if that's going to be two years old, then maybe not. If her shirt size is smaller than her age, the Britaxes may be just fine and the NextFit overkill. I'd suggest heading to Babies R Us or Buy Buy Baby to play with both.
Also, what I'd probably do is pass down the Marathon you have, then buy one convertible for the baby and one more combination seat for your older daughter. That way instead of buying two $280 seats you buy one and then a $150 seat. Save you a bit of money and get your kids perfectly safe.
Wendy