I agree with Jennifer. If your oldest child's seat is not expired then use it for the baby and get a high weight harness seat for the older child. Here is a list of seat that ladies on another board I visit like a lot.
Both of the seats that you listed have low top harness slots so are not going to last either of your kiddos to booster age unless they are just super duper tiny. If you do want to get both kids new seats, VERY affordable options are the FPSVD and the Evenflo Triumph Advance. I'd consider them for both kids. Both seats will keep most kids harnessed until they are 5 to 6 years old. The Apex for your oldest (assuming she is ff) as long as you have the adjustable headrests that it requires.
United States)
Convertible Seats
Britax Marathon, Decathalon, or Boulevard (MA/DC/BV). 17 in top slots, RF 5-33lbs, FF up to 65lbs in 5 point harness. Awesome seats, allow for rf tethering, has lockoffs to replace locking clips, very easy installs, many bells and whistles that make them easier to use correctly every time, lasts most kids to age 5-6y/o. 220+ Check the FAQ sticky thread for a description of the three.
Fisher Price Safe Voyage Deluxe (FPSVD).17 inch top slots, RF 5-33lbs, FF up to 55lbs in a 5 point harness. Made by Britax, sold under the FP name- basically a stripped down Britax Marathon. Great seat, awesome price $129 (regular 199$), and will last most kids to age 5-6 y/o.
DISCONTINUED DUE TO POOR SALES-- As of right now Britax is still manufacturing them & new dates have been shipping.
New Evenflo Triumph Advanced RF to 35lbs; FF to 50 lbs. Max harness height of 17". The new Triumph has the infinite harness adjuster too.. no rethreading the harness! Much nicer than the old Triumph! Much taller shell than the old Triumph too! Priced at 150$ VERY new seat so not in many stores yet I dont think. We think it will be small enough for newborns!
Sunshine Kids Radian. 18 in top slots, RF 5-33lbs, FF up to 65lbs in 5 point harness. Great seat, only second to Britax because it has some compatibility issues in some vehicles when installed with seatbelt. Really good seat otherwise, allows for RF tethering, lasts most kids to 6-7y/o. 8yr lifespan (most have a 6yr) $175+
www.skjp.com has the manual available for viewing.
Recaro Como & Signo. Recaro makes seats for NASCAR and has made car seats for the European market and a few for the US market for years. RF to 35lbs, forward face to 70lbs, and has 19" top slots. Bottom slots of 9" so not ideal for newborns. Allows for rear facing tether!
The outside width of the seat at its widest point is approx. 19 ½ to 20.
The Como & the Signo are basically the same shell the biggest difference is that the Signo has an infinitely adjustable harness (where the Como has to be re-threaded) and the Signo's latch straps are on a bar - similar to Britax (on all their models) - so you don't have to re-thread the latch belt if you're going from rear facing to forward facing.
Como is $249; Signo is $289
Como is set at 2nd week of August
Signo is 3rd week of September
And lower cost Convertibles we love... (For primary or secondary cars... they are still GREAT seats)
Cosco Scenera $40
5pt harness version.. Very basic no frills seat. Minimal padding, but it's rare for a LO to complain about it. 35lb RF limit with a taller shell! 40lb FF limit however most kids outgrow it in torso height at about 3-3.5yo due to the low 15in top slots.. It just doesnt offer enough torso grow room to get most kids to 40lbs. Readily available at Wal*mart/ Target/ K-mart. Usually requires 1-3 chunks of pool noodle to get a good recline.
-Safety First Uptown: 35rf/ 40ff/ tall 16.5in top slots so this is the ONLY RF/ff 40lb limit convertible that will likely get a kid to 40lbs. EPP foam, velvety plush cover, super cushy. Very easy to use & install. 8 yr lifespan! (Most have a 6 yr) Around $100 at albeebaby.com, Target.com, and in store at Wal*mart.
Here's an awesome thread comparing the Scenera & Uptown on carseat.org. Side by side pictures.
http://www.car-seat.org/showthread.php?t=10324
The Graco Comfortsport, Evenflo seats, and the Alpha Omega 3in 1 types are very small seats that are usually outgrown the same time as the 40$ Scenera. You dont get much for your money. While they are safe there are just other options that offer more for the same cost.
Forward facing only seats (child must be over age 1, and 22#s)
Safegaurd GO. Harnesses to 60#s, becomes a no back booster after. Top slots are 17 to 17.5 inches, needs adjustable headrest behind it for support. Requires top tether anchor. Child must be 30#s to use. Retails for $179
Britax Regent (used to be Husky). KING of car seats is right! Harnesses to 80#s. Top slots are 20 inches. Requires a top tether after child is 50#s. Retails for $220-$260. Not good for children under 2yo. Requires college degree to read the manual.
Ride Safer Vest. A vest that comes in two sizes, Small 35-60#s, Large 50-80#s. The vest can be used with lap only belt. Requires top tether anchor & headrest. Retails for $89-$99. Great for 3 across situations in small vehicles! Tether strap HAS to be used if child is under 6 yrs old.
http://www.safetrafficsystem.com/
FF/Harness Combo Harness/Booster Seats
Cosco/Safety 1st Apex. Only higher weight Combo (FF harness/Booster) seat that harnesses to 65#s. Top slots are 17 inches high. Requires adjustable headrest or a very tall vehicle seatback behind it for support. Then becomes a very nice BUT VERY WIDE booster that doesnt fit small children well.. This seat is outgrown totally when the tips of the childs ears reach the top of the vehicle seatback. (Fully extended it has a back height of 32.5in tall) The new blue/black and all the brown ones have EPP Foam in the headrest. Retails for $99-150.
Evenflo Generations/Bolero: 80-110$ depending on model. FF ONLY to 40lbs with harness. Great harnessed seat/ great booster mode. Open belt guides. TALL 16.5in top slots. Adjusts tall in booster mode too! Flip up armrests make kids happy & make it easier to use in booster mode too!
(The Bolero is the dressed down version of the Generations. It lacks the piddle pad).
Evenflo Chase & Graco Cargo. Harness to ONLY 40lbs ff, 17in top slots. Makes ok, at best, booster. Smaller closed belt guides.. but this can be ok if your seat belts lock & you lock them every time. The Chase is about 60$. The Cargo with front adjuster is about 90$.
The Chase is good for kids/parents that keep forgetting that chest clips go on the chest!
Avoid most other combo seats... VERY low top slots or seatback heights all with 40lb limits.. Just not worth the cost. This includes all the Eddie Bauer, Cosco, Dorel, and 3 in 1's. While safe & decent seats the harness mode doesnt offer much grow room.
