I'm a Child Passenger Safety Technician and I have used both these seats.
Firstly, they're both wonderful, and I would recommend either of them without reservation. That being said, they both have their little lists of pros and cons.
The Nautilus has a 65 lb limit on the harness while the Frontier has an 80 lb limit. However, they have identical top harness heights. Both seats will be outgrown before most kids reach 60 pounds, because a seat is outgrown by height when the shoulders reach the top harness position.
The Nautilus has a 6 year expiration as a harnessed seat and a high back seat, and the backless booster portion has a 9 year expiration.
The Frontier has a 9 year expiration overall, and does not convert to a backless booster, however, as a high back booster it adjusts higher than any other booster on the market today, and will fit most adult women comfortably.
The Frontier is LATCHable in booster mode, the Nautilus is currently not (although Graco has flipflopped on this a few times).
The Nautilus has a wonderful cupholder, and the Frontier's cupholder is middling.
The Nautilus has four harness positions and the Frontier has nine, this means the Frontier will get you a more precise harness fit more of the time than the Nautilus.
The Nautilus is EXTREMELY easy to install in almost all vehicles, and the Frontier has some definitely installation quirks.
The Nautilus requires 100% of the base to be on the vehicle seat, and the Frontier requires only 80%, so the Nautilus isn't a good fit for pickups with small back seats.
Now that you are moving your 14 month old to the Marathon, remember that she should still be rearfacing. We don't recommend forward facing at 1 and 20 anymore. Most kids should fit rearfacing till around 3 to 4 years depending on size.
Also remember that if you bought the Marathon when your son was a baby, it will expire in three years or so. :)