The Comfortsport is a huge waste of money. most kids outgrow it by their 2nd birthdays in height! It has the shortest shell on the market and the top harness slots are a full 3-6" SHORTER than other seats on the market for less money. Most car seat experts call it the 'crappysport' for those reasons. They are so not worth they money!
you didn't say how old your child is ,but since his full time seat is a Nautilus, I'm guessing your child is over 2/3 yrs old. The Scenera fits most kids well to age 3-4, so you should check your child in the seat at the store before buying it. my oldest outgrew the Scenera at age 3, my youngest was able to use it to age 4 as our travel seat. It will harness to 40lbs and the top harness slot is ok. So check yoru child before buying.
The Nautilus can be taken on the flight, so instead of buying a new seat your son can't use for long, bring the Nautilus and strap him right into the airplane like the car. He will be more comfortable, and you will have a seat when you arrive at your destination.
NEVER check a car seat. The damage done to a car seat while stowed with luggage under the plane is the same that is sustained in a collision in the vehicle. The seat will not be treated with care, it will have bags and other luggage thrown on it, and it will be shifted and thrown around during flight. NEVER CHECK A CAR SEAT!
So bring his seat, strap him in on the flight and bring it in the rental with you. FAmiliarity is great while on vacation, nad he will be safe. No extra money spent. But if you do insist on buying another seat, avoid the Comfortsport. The Scenera is great, but again try him in it before buying it, and DO NOT CHECK IT WITH BAGGAGE! bring it on the plane.