My four year old is still in his stage 2 seat but when my dd was 4 I had her in a princess booster seat from mothercare which wasn't too expensive and uses the main seat belt. It has head support for long journeys when she falls asleep. Ds2 will need the next seat too soon so watching this thread with interest for ideas.....
I haven't got to a booster yet, but the other car seats we have bought have been isofix, and I wouldn't buy anything else,
they are so solid i.e. just don't move, whereas other a non-isofix seat i have come across seems to wriggle around far too much for my liking. and you always know that it is fitted correctly, as they are so easy to fit.
I've just bought a Britax Evolva Isofit for £95 from babyworld's website. I've just checked and bambinodirect are also selling it for £90 Having always used Isofix I'd personally never buy anything else and this came top in Which tests.
Personal opinion but I have 2 children, ds2 is 2 and in britax evolva 123 (not isofix) and my husband is the only one who can get that seat to anchor well. Ds2 is still using the 5 point harness.
Ds1 is 5 and is in a maxi cosi rodi highback booster with side impact wing things which according to Which magazine was one of the safest at the time we bought it and they rated isofix as the best.
BUT on the maxi cosi the seatbelt is difficult to pull through the guide, we knew this when we bought it but didn't think of the implications. When ds1 leans forward the belt loosens but when he sits back it doesn't automatically re-tighten, he has to pull it back tight.
He is almost 6 so this isn't a problem but we have said that the next car seats we buy will be isofix because our car has the isofix fittings (we bought the car after we had bought the car seats)
Isofix is probably more suited to the younger children's child seats as it guarantees the fitting. However, in the event of a crash I would rather my child is bolted to the car with the seat rather than just the seat belt. It is partly irrational as I am only held in with a seatbelt
I would rather pay a bit more money to know that I have done everything to make my children as safe as possible.
I would only use rear facing seats for my children (if they ever happen - TTC) but that is because even tho i am skilled at seat fitting - well i should be after 20years caring for babies and toddlers - some just work slack, some wobble no matter what and i do feel happier in the event of an accident with the seat taking the major initial force rather than their body.
The drawback in my veiw of using an ISOFIX seat with just a belt is the addition weight that the belt and so the child will bear in a sudden deceleration.