Hi S.,
I'm assuming your 1st is going to outgrow his/her seat soon....to safely keep your kiddo harnessed in a 5pt (which is by far much better than moving into a belt positioning booster too early) I'd suggest either the Britax Marathon/or Radian 65 both go to 65 pounds, the Britax Regent (or the new Britax Frontier coming out this summer- this looks wonderful, it will harness to 80 pounds then become a belt positioning booster, the Graco Nautilus is new and getting great reviews, it harnesses to 65 pounds and then becomes a good booster. The Cosco Apex 65 is okay, but needs high seat backs/head rests to be used. Depending on how old your first child is I would recommend certain seats over others.
You child should continue to be in some type of seat most likely until they are about 10 years, 4'9'' is the government guidelines....that being said, keeping them in a 5pt harness is the safest option, moving to a high back booster is only recommended for children who are at least 5, well over 40 pounds and are able to SIT STILL, and not play with the belt every single time.....seat belts and young children in an accident don't always get along...which is why the belt positioning boosters are important to properly spread out the impact forces...I plan on keeping my DD in her Marathon until next year when she's about 5 1/2 then she'll go to a seat like the Regent that is a 5 pt harness to 80 pounds- and I don't care that all of her friends at age 4 are in backless boosters... I know she is safe and that is what matters, not the convenience of not having to strap her in (I've heard that a lot- well, how about the inconvenience if your child's in an accident and has permanent injuries.....
Sorry so long but I feel that too many parents (I'm not implying you S.) don't ask these questions and just read the boxes at their local store without doing a little research...those low back boosters that say 30- 100 lbs are not appropriate for 30 pounders!
Great question! Email me if you want ideas where to buy online that is reliable/cheapest.
B.