The Cosco Summit is actually quite a wide combination seat

What is your vehicle year, make, model?
Safest practice means puting the least protected person (regardless of age) in the center-most position when that seat accomodates the appropriate carseat/
booster....
RF is safer than
FF, harness is safer than
booster: if you have a center shoulder belt then your
booster'd kiddo should be in the center.
Also remember that boosters require at least 32" between the front seatback & the backseat.
The
Radian fits infants well & keeps bigger kids harnessed longer than most carseats. It is generally more difficult to install, but a good CPST shouldn't have a problem unless there is a real compatability issue. It folds for travel & would also keep your 35# 6 year old safer

At the very least you might consider a more dedicated
booster for your bigger kiddo use if the center does not have shoulder belt: such as
Britax Parkway (currently on sale at Sears) or anything with similar deep head wings (the page titled "Preschool -- Preteen" in my website explains this issue in more detail)
Any carseat that fits both the child + vehicle & will always be used correctly is very safe. A seat with fancy features that is used incorrectly will not be as safe.