You should call your local CHP & see what they recommend. When my son was just over 2 yrs I was really tired of the Britax Roundabout.....just too heavy & cumbersome to move between our 2 cars so I bought a seat that you could take out the 5 pt. harness & make it a booster seat & use my car's seatbelt. BIG mistake! It was made by Evenflo & cost about $60 +/-. When it came time to take out the harness, the seatbelt slots at the shoulders of the carseat hit him wrong. So, we bought a Graco booster seat. Here's my 2 cents.....unless necessary, keep them in that Britax until it's time for a true booster seat when they hit 40 lbs. Are their ears above the top of the seat? This is why I say call your local CHP....they install & do carseat checks for free. So, make an appt. & have them look at the kids in their current carseats & ask their advice. When I did this, they highly recommended all carseats by Graco. They make a seat for about $70 or so that has the 5 pt harness. You can later remove the harness & use your car's seatbelt. Those Britax seats you mentioned are very expensive (over-priced, too, if you ask me), they're big & bulky & don't fit in all makes of cars. They're also not easy to travel with. Since we had our first son 7.5 yrs ago, baby/kids gear has gotten more & more expesive & so much of it really is not a dire necessity but marketing/commercials makes us feel it is so we fork out the money that could have been better spent elsewhere. And in your case, you're buying 2 of them. Do some serious investigating, ask your friends & the CHP before you spend all that money on 2 more carseats. Good luck!