Maybe I could help?
The 9 months to 11 years seat will usually not recline - I know of one that does, but it only goes back a little way (Britax Evolva 1/2/3) and you must remove the seatbelt and re-fit if you want to change between the two.
The 9 months to 4 years seat will usually have at least 3 reclining positions (usually 4 or 5) and is specifically designed for the smaller child. It is higher which allows smaller ones to see out of the car window.
The 9 months to 11 years is the better buy, but you may have to compromise on some features especially if you are wanting to use it from 9kg. Try both types in your car with your little one in it and see for yourself.
Hope that helps!

Could someone please advise me why I shouldn't buy forward facing, high back booster seats suitable for children between 9kgs and 36kgs ie 9 months- 11yrs? I am confused between which car seat is more appropriate. Why shouldn't I purchase the "with harness" (9months - 11 years) rather than spend double the money on a forward facing booster seat (9months- - 4 years)and then purchase a high back bosster seat "without harness" (4 -11 years) Confused