Hello, welcome & CONGRATS!
I suggest visiting your local Babies-R-Us: they have a nice selection of seats & they offer discounts for parents of multiples
The seats I would consider include....
- Recaro Signo/Como -- to be released within a couple weeks, this company is very well respected (as designers of racing seats, etc.) These seats have the highest RF & FF limits, both. They can also be tethered RF & should be quite comfortable based on their other carseats. They also supposedly have nice ease of use features.
- Britax Marathon/Bouelvard -- Britax is another well respected name because they also perform more extensive crash testing & have long been considered "innovators of the industry". These seats are easy to install in most vehicles & easy to use, they keep kids harnessed until truly ready for a booster. Very few kids complain about comfort in these seats & they have lots of cover options to choose from.
- Sunshine Kids Radian65 -- a relatively new product that has a bit of a learning curve for installation, but is not as expensive as the above options yet has many similar safety features. Kids can stay harnessed slightly longer in this than Britax. It's very narrow & folds for travel.
- Evenflo Triumph Advance -- the improved version of the older Triumph is easier to use, has higher RF limits & fits FF about as long as the Marathon/Boulevard. This seat is very well padded for comfort & the less expensive.
- Safety 1st Uptown -- similar the basic, well loved Cosco Scenera, it is lightweight with the same high RF limit, but has EPS foam (Scenera is just about the only seat that doesn't yet) as well as more comfort padding. This seat is the least costly but also has the lowest FF limits.
I personally would consider the Uptown as spare seats & try for any of the others based on that which you find the easiest to use correctly. (The less often you have to uninstall/reinstall seats the better because frequent moving around can increase risk of misuse). Remember that it must fit the vehicle correctly as well as the child
Once you've got your seats & your kids, call 1-866-SEATCHECK to find your local technician to check your install/use.