Booster for 8.5-9 year old

[Piglet]

In light of all the threads about harnessing vs. boosters for 40+lb kids, I am thinking of transitioning DS2 to a booster in the next year. He is not ready yet and he has an inch of room above his shoulders in the MA, so no rush. In the meantime, DS1 is in a 4 year old Parkway in my van. I love the Parkway and think it would be a good transition seat for DS2.

What about DS1? What is a good booster for a big kid? He is thin but average height. Should I look at the backless ones? He is in a backless in the grandparents' cars and honestly he rides with them more than he does with us (MIL and my dad takes the kids to school and back). We have the Cosco Highrise and the Evenflo Big Kid backless in their cars. Are these as good as anything else or would I benefit from the Clek one? I am in Canada, if that makes a difference.

[Joolsplus2]

That's a fine age to be in a backless. Ideally you'd only go backless in a car with curtain airbags, though, because highbacks offer so much head protection in side impacts.
Cleks are sturdy, well built little boosters. Kind of short for thigh support and my kids either need a plastic footstool to prop their feet on or their heels rest on the front of the seat cushion. But they LOVE the armrests! I might prefer to get the Monterey so it could be used highback or backless and LATCHed. Or if you have the Combi Dakota (don't think you do in Canada...), that's my kids' favorite.