I have only just (in the past couple of months) put my 8 year old on a backless booster (he's 132cm tall). I'm actually using a Trunki Boostapac, for a couple of reasons, both related to the seatbelt he uses (he's in the middle seat of a Peugeot 5008):
- The Trunki is 'thicker' than all the other booster cushions I've seen so it raises him up higher
- The Trunki has a red clip that you feed the seatbelt through and this positions the seatbelt really well across his chest and shoulder.
The middle seatbelt in the 5008 comes down from the roof too far forward and at too steep an angle for my liking. When DS sat on a booster cushion that didn't have a shoulder guide the seatbelt wasn't even touching his body. I could have slid the seat he was sitting on forward, but then DD2's Multi Tech ERF seat was getting in the way of him fastening the seatbelt (I have 3 children, DD1 is on a Britax KidFix on the other side of DS). The joys of trying to fit 3 children in the back of a car!
The only other booster cushion I could find with a seatbelt guide was the Graco one, but it's too wide for my car. It was covering the seatbelt buckles (they are recessed into the seat).
Also, I came across this on the Rear Facing facebook page www.researchgate.net/profile/Katarina_Bohman/ I think it basically says that after a certain age there is a question about how effective a HBB is - due to the way children sit or slouch or lean out of the seat. A booster cushion is just as effective for a child who is nearing the height limit for needing a car seat.
I don't think I'm 100% happy with him on a backless booster (when we were travelling over Xmas and New Year I put him back in a HBB due to it being a lot of motorway travel), but he's in the middle of the car so won't get any direct impact from a side-on accident should the worst occur.
To answer your question though, I have 2 Britax Adventure HBBs (that I use if I need to put 2 HBBs in the car) and the aforementioned KidFix that I bought for DD1 when I got the Trunki for DS. Not sure why I moved her to the KidFix, I think I just liked the fact you don't have to bother fastening the seatbelt over the empty seat. I like both different HBBs equally, maybe you should go for the Adventure since it's usually on offer for about £30 and you won't need another 5 years use out of it (unless you will be passing down to a younger child)