OK, I have finally bought the Maxi Cosi Priorifix.
I have the following to report :
1. The installation is not as easy as I was led to believe because the limited rear space of the car does not give you much room to move around and the fact that this seat is bloody heavy!
2. Because of the small headroom of the IS model, I suspect my kid's head will hit the roof before he is one year old (he is currently 5 months old).
3. This car seat has a support leg and so I had been worrying sick when I tried to move the seat inside the car that I might accidentally hit the paintwork of the car with the support leg.
Moral of the story, try other car seat without a support leg (like Brittax). I also think that for a car with limited rear space like the IS, installation would a lot easier by using the seat belt rather than the ISOFIX thingy.