High back booster or new 25kg harness seat for 4 year old?

[MelinaMercury]

DH has just passed his driving test and bought a car (07 Megane), 3.11yo DD is 16.5kg so is about to outgrow her group 1 seat.

As we very rarely travel by car she's not used to seatbelts and refuses to use them and unclips them if I try to use it on the bus so in not sure I trust here with a standard belt just yet.

I'd like something like the Britax TWE but DH thinks a decent HBB would be a wiser investment.

She's got until 18kg before she is too heavy and at this age weight gain is slower so you have a while yet.

When she is ready to move I personally would use a hbb but others may have a different opinion